Free CDL Practice Test

Florida Air Brakes CDL Practice Test

Practice air brakes CDL questions for Florida. Review each question, learn from the explanations, and prepare for your CDL knowledge exam.

Question 1 of 125

What is the purpose of the air compressor in an air brake system?

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What is the purpose of the air compressor in an air brake system?
- A. To apply the brakes
- B. To pump air into the air storage tanks to build and maintain brake system pressure
- C. To cool the brake drums
- D. To release the spring brakes
Answer and explanation
Correct answer: To pump air into the air storage tanks to build and maintain brake system pressure
The compressor pumps air into the storage tanks to replenish the braking supply. The governor controls when it pumps; the tanks store the air used by the brakes. A conventional compressor may be engine-driven, but this question does not prescribe every vehicle design.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What does the air compressor governor do?
- A. Controls the spring brake release pressure
- B. Controls when the compressor pumps air — cutting in at around 100 psi and cutting out at 125 psi
- C. Monitors brake temperature
- D. Controls vehicle speed
Answer and explanation
Correct answer: Controls when the compressor pumps air — cutting in at around 100 psi and cutting out at 125 psi
The governor starts and stops compressor pumping as tank pressure changes. The CDL manual gives about 100 psi cut-in and 125 psi cut-out as examples; use the vehicle manufacturer's specifications for the actual settings. Unloading the compressor is not the same as stopping the engine.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
For manually drained air tanks, how often does the CDL manual call for draining?
- A. Only when the warning light comes on
- B. Only at scheduled maintenance intervals
- C. Once a month
- D. Daily — water and oil build up in the tanks and can freeze in cold weather, causing brake failure
Answer and explanation
Correct answer: Daily — water and oil build up in the tanks and can freeze in cold weather, causing brake failure
For tanks with manual drains, remove the accumulated water and oil at the end of each working day. Automatic draining is a different arrangement and must be checked for the fitted equipment. Warm weather does not eliminate moisture contamination; do not infer a unique state-specific draining rule.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the purpose of the safety relief valve in an air brake system?
- A. To control air pressure to the brake chambers
- B. To drain water from the tanks
- C. To release air if pressure in the tanks gets too high (usually set at 150 psi) — preventing tank rupture
- D. To apply the brakes in emergencies
Answer and explanation
Correct answer: To release air if pressure in the tanks gets too high (usually set at 150 psi) — preventing tank rupture
The relief valve protects the reservoir and system against excessive pressure. The manual gives 150 psi as the usual setting. Air release from this safety valve indicates a fault to have fixed; it does not by itself identify the only possible failed component.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A dual air brake system has two separate air circuits. What is the purpose of this design?
- A. To retain some braking through the other circuit if one loses pressure; stopping distance increases
- B. To increase braking power on highways
- C. To allow two drivers to control the brakes simultaneously
- D. To reduce air consumption
Answer and explanation
Correct answer: To retain some braking through the other circuit if one loses pressure; stopping distance increases
A dual system has separate air circuits operated by one set of controls. If one system loses pressure, the remaining brakes may provide reduced braking and stopping distance increases. Stop safely and have the fault corrected; redundancy is not permission to keep driving with a failed circuit.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What are the three types of air brake systems?
- A. Front, rear, and trailer brakes
- B. Manual, automatic, and electric
- C. Drum, disc, and hybrid brakes
- D. Service brakes, parking (spring) brakes, and emergency brakes
Answer and explanation
Correct answer: Service brakes, parking (spring) brakes, and emergency brakes
Air brake systems have three components: service brakes (applied with the brake pedal during normal driving), parking brakes (spring brakes applied when air is released, via parking brake control), and emergency brakes (spring brakes that apply automatically if air pressure drops too low).
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What should you check first during an air brake pre-trip inspection?
- A. Check the brake pedal position
- B. Build air pressure to governor cut-out, then check for leaks with the engine off — should not drop more than 1 psi per minute
- C. Listen for air escaping only
- D. The air pressure gauges only
Answer and explanation
Correct answer: Build air pressure to governor cut-out, then check for leaks with the engine off — should not drop more than 1 psi per minute
Pre-trip air brake check: 1) Build air to governor cut-out (~125 psi), 2) Turn off engine, 3) Check gauges — pressure should not drop more than 1 psi per minute with no brakes applied, or 3 psi per minute with brakes applied. More loss indicates a significant air leak requiring repair.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When testing the low air pressure warning during pre-trip, you should:
- A. Fan the service brakes to reduce pressure to about 60 psi — the warning light and buzzer should activate before pressure drops below 60 psi
- B. Wait until the warning activates on its own during driving
- C. Only check warning lights on the dashboard
- D. Test it monthly, not at every pre-trip
Answer and explanation
Correct answer: Fan the service brakes to reduce pressure to about 60 psi — the warning light and buzzer should activate before pressure drops below 60 psi
To test low air warning: build air to cut-out pressure, then repeatedly apply and release the brakes to reduce pressure. The low air warning (light and/or buzzer) must activate before pressure drops below 60 psi. If it doesn't activate in time, do not drive the vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When should spring brakes automatically come on?
- A. Any time you press the brake pedal
- B. When air pressure drops to approximately 20-45 psi in a dual air system
- C. When engine oil is low
- D. When you engage reverse gear
Answer and explanation
Correct answer: When air pressure drops to approximately 20-45 psi in a dual air system
Spring brakes automatically apply when air pressure drops to approximately 20-45 psi (varies by system). At this pressure, the spring force overcomes whatever air remains and applies the brakes. This is the fail-safe feature of air brake systems.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
How do you check that spring brakes are working during pre-trip inspection?
- A. Ask a mechanic to check them
- B. By pressing the accelerator and feeling resistance
- C. Run the vehicle on a grade and see if it holds
- D. Build air pressure, turn off the engine, push the trailer air supply valve to close it, then reduce pressure to 20-45 psi — spring brakes should apply fully automatically
Answer and explanation
Correct answer: Build air pressure, turn off the engine, push the trailer air supply valve to close it, then reduce pressure to 20-45 psi — spring brakes should apply fully automatically
Spring brake test: With engine off, step on and off service brakes to reduce air pressure to 20-45 psi range. The spring brakes should automatically apply, holding the vehicle stationary. If they don't apply automatically at this pressure, the system needs maintenance.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is a 'pushrod stroke' and why is it important to check?
- A. How far the fuel pump arm moves
- B. The distance the steering wheel travels
- C. How far the brake chamber pushrod moves when the brakes are applied — if over the adjustment limit, brakes are less effective
- D. How far the clutch pedal depresses
Answer and explanation
Correct answer: How far the brake chamber pushrod moves when the brakes are applied — if over the adjustment limit, brakes are less effective
Pushrod stroke (brake adjustment) measures how far the brake chamber rod moves when air is applied. If the stroke exceeds the adjustment limit (varies by chamber size), the brakes are under-adjusted and less effective. Out-of-adjustment brakes are an out-of-service violation.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The correct way to brake with an air brake system in normal conditions is:
- A. Apply maximum pressure immediately
- B. Use only the hand control valve
- C. Apply smooth, steady brake pedal pressure — maintain control and do not overheat the brakes
- D. Pump the brakes rapidly like hydraulic brakes
Answer and explanation
Correct answer: Apply smooth, steady brake pedal pressure — maintain control and do not overheat the brakes
For normal air brake operation: apply smooth, steady pressure to the brake pedal. This provides controlled braking without locking wheels. Save hard braking for emergencies — smooth braking is more effective and maintains vehicle control.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When driving on a downgrade with air brakes, what is the 'snub braking' technique?
- A. Apply service brakes hard enough to feel the deceleration, then release when speed drops by about 5 mph — repeat as needed
- B. Holding the brake pedal at exactly 50% pressure
- C. Continuous gentle brake pressure
- D. Only use trailer brakes on downgrades
Answer and explanation
Correct answer: Apply service brakes hard enough to feel the deceleration, then release when speed drops by about 5 mph — repeat as needed
Snub braking on downgrades: apply brakes firmly until speed drops 5 mph below your target speed, then release brakes to let them cool. Repeat this cycle. This prevents brake overheating (brake fade) that results from riding the brakes continuously.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What speed should you set as a target when beginning a downgrade with air brakes?
- A. The same speed as on flat ground
- B. 5-10 mph below the speed you want to maintain on the grade, so when you release brakes you have a speed buffer
- C. Any speed is safe with air brakes
- D. The maximum posted speed
Answer and explanation
Correct answer: 5-10 mph below the speed you want to maintain on the grade, so when you release brakes you have a speed buffer
Before starting a long downgrade, slow to 5-10 mph below your target descent speed. This gives you a speed buffer. As you apply snub braking and the vehicle accelerates back up, you apply brakes again. This pattern prevents overheating.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What happens if you hold the brake pedal down continuously on a long downgrade?
- A. The spring brakes activate automatically
- B. Nothing — air brakes are designed for continuous use
- C. The brakes overheat, causing brake fade or failure — you lose the ability to slow the vehicle
- D. The air pressure builds up, causing surging
Answer and explanation
Correct answer: The brakes overheat, causing brake fade or failure — you lose the ability to slow the vehicle
Continuous brake application generates extreme heat in drum brakes on long downgrades. As drums overheat, the brake lining material loses friction (brake fade), and braking effectiveness is drastically reduced. This is one of the most dangerous situations in large vehicle operation — use snub braking instead.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the purpose of brake drums?
- A. To provide the friction surface against which brake linings press to slow and stop the vehicle
- B. To house the wheel bearings
- C. To monitor tire temperature
- D. To store air for the brake system
Answer and explanation
Correct answer: To provide the friction surface against which brake linings press to slow and stop the vehicle
Brake drums rotate with the wheels. When brakes are applied, brake shoes press outward against the inside of the drum. The friction between the shoes and drum slows the rotation of the wheel, slowing the vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What does an S-cam do in a drum brake system?
- A. Spreads the brake shoes outward against the drum when the brakes are applied
- B. Monitors air flow to the brake chambers
- C. Controls air pressure
- D. Adjusts the parking brake
Answer and explanation
Correct answer: Spreads the brake shoes outward against the drum when the brakes are applied
When air enters the brake chamber and pushes the pushrod, the slack adjuster rotates the S-cam. The S-cam spreads the brake shoes outward against the drum surface, creating friction to slow the vehicle. This is the fundamental mechanism of air drum brakes.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the purpose of slack adjusters?
- A. To regulate air flow rate
- B. To adjust air pressure
- C. To keep brake shoe-to-drum clearance within proper limits — maintaining consistent pushrod stroke length
- D. To control brake chamber size
Answer and explanation
Correct answer: To keep brake shoe-to-drum clearance within proper limits — maintaining consistent pushrod stroke length
Slack adjusters (manual or automatic) maintain the correct distance between brake shoes and the drum. As brake linings wear, the drum-to-shoe gap increases. Slack adjusters compensate for this wear, keeping pushrod stroke within adjustment limits.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is a 'wedge brake'?
- A. A type of drum brake where air-cylinder pushrods drive a wedge to expand brake shoes
- B. A parking brake type only
- C. A brake used only on trailer tandems
- D. A hydraulic brake component
Answer and explanation
Correct answer: A type of drum brake where air-cylinder pushrods drive a wedge to expand brake shoes
Wedge brakes are a type of air drum brake where the air chamber pushrod drives a wedge directly between brake shoes to spread them against the drum. They are used on some CMVs as an alternative to S-cam brakes. They require regular inspection and may have automatic adjusters.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Disc brakes on commercial vehicles offer which advantage over drum brakes?
- A. Better heat dissipation resulting in more consistent braking and reduced fade on long downgrades
- B. They are cheaper to manufacture
- C. They require no maintenance
- D. They are lighter in weight
Answer and explanation
Correct answer: Better heat dissipation resulting in more consistent braking and reduced fade on long downgrades
Disc brakes dissipate heat much more effectively than drum brakes because the rotor is exposed to air on both sides. This reduces brake fade on long downgrades and provides more consistent braking performance in demanding conditions.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
During a pre-trip inspection, brake drums should be checked for:
- A. The manufacturing date
- B. Cracks, especially at air holes — cracked drums are an out-of-service condition
- C. The brand name
- D. Color and finish
Answer and explanation
Correct answer: Cracks, especially at air holes — cracked drums are an out-of-service condition
Inspect brake drums for cracks — particularly at the air holes in the drum surface. A cracked drum can fail catastrophically under the heat and stress of braking, causing sudden brake failure. Cracked drums are an immediate out-of-service condition.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the minimum thickness for brake lining before it must be replaced?
- A. Linings must be replaced when they feel thin
- B. 1/4 inch for shoe brake linings not adjacent to drums; varies by position
- C. Any thickness is safe if the brakes are working
- D. 1/8 inch for all locations
Answer and explanation
Correct answer: 1/4 inch for shoe brake linings not adjacent to drums; varies by position
FMCSA out-of-service criteria for brake lining thickness: less than 1/4 inch for non-shoe brake linings, or less than 1/8 inch for shoe brake linings not adjacent to the drum. Linings below these thresholds are an out-of-service condition.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What happens when you push the 'Trailer Air Supply' valve in (or pull it out, depending on the vehicle)?
- A. Pushing in supplies air to the trailer; pulling out stops air supply and applies the trailer emergency/spring brakes
- B. The trailer service brakes apply
- C. It activates the trailer ABS system
- D. It only controls trailer lights
Answer and explanation
Correct answer: Pushing in supplies air to the trailer; pulling out stops air supply and applies the trailer emergency/spring brakes
The Trailer Air Supply (TAS) valve controls air supply to the trailer. When closed (pulled out), it cuts off air to the trailer and causes the trailer spring brakes to apply. It is used to test the trailer brakes and to park with trailer brakes applied.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The 'hand valve' or 'trolley valve' in the cab controls:
- A. The tractor service brakes only
- B. The emergency brake system
- C. The trailer service brakes — used primarily for testing and modulating during backing
- D. The spring brakes
Answer and explanation
Correct answer: The trailer service brakes — used primarily for testing and modulating during backing
The hand valve (trolley valve) applies the trailer service brakes independently of the foot pedal. It is primarily used to test the trailer brakes during pre-trip inspection. Using the hand valve alone while driving is generally not recommended as it can cause jackknifing.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the purpose of air dryers in an air brake system?
- A. To regulate temperature in the cab
- B. To cool the compressed air
- C. To remove moisture and contaminants from compressed air before it enters the storage tanks
- D. To increase air pressure
Answer and explanation
Correct answer: To remove moisture and contaminants from compressed air before it enters the storage tanks
Air dryers remove water, oil, and contaminants from compressed air. Without an air dryer (or with a failed one), moisture accumulates in air lines and tanks. In cold weather, this moisture can freeze and block air lines, causing brake failure.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
During a pre-trip inspection with the key on and engine off, the air pressure should build from 85 to 100 psi within how many seconds?
- A. 45 seconds
- B. 120 seconds
- C. 10 seconds
- D. No time limit — any rate is acceptable
Answer and explanation
Correct answer: 45 seconds
A healthy air compressor should build pressure from 85 to 100 psi within 45 seconds with the engine at operating RPM. If it takes significantly longer, the compressor or governor may need service. Slow pressure buildup can indicate a leak or worn compressor.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When checking air hoses and lines during pre-trip inspection, you should look for:
- A. Only end connections
- B. Color variations only
- C. Chafing, cracking, bulges, improper connections, and any signs of wear that could lead to failure
- D. The manufacturer's label
Answer and explanation
Correct answer: Chafing, cracking, bulges, improper connections, and any signs of wear that could lead to failure
Air hoses should be free from chafing against other components, cracks, bulges, and improper or loose connections. A failed air hose can cause sudden, complete loss of air supply to brakes, with potentially catastrophic results.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why should you not 'fan' your brakes (rapidly pump them on and off) in an air brake-equipped vehicle?
- A. It increases brake wear
- B. It is an FMCSA prohibited technique
- C. It causes the ABS to activate incorrectly
- D. Fanning depletes air reservoir pressure faster than the compressor can replace it, potentially causing brake failure
Answer and explanation
Correct answer: Fanning depletes air reservoir pressure faster than the compressor can replace it, potentially causing brake failure
Unlike hydraulic brakes (which benefit from pumping), rapidly applying and releasing air brakes 'fans' the system, depleting air faster than the compressor can rebuild it. If pressure drops below 60 psi, you lose effective braking. Use smooth, steady brake pressure instead.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What role do air storage tanks play in the air brake system?
- A. They replace the brake chambers
- B. They store compressed air for immediate use when brakes are applied, since the compressor cannot supply enough air instantly
- C. They apply the brakes directly
- D. They control air pressure on downgrades
Answer and explanation
Correct answer: They store compressed air for immediate use when brakes are applied, since the compressor cannot supply enough air instantly
Air tanks store compressed air for the brakes. The stored reserve allows several applications even if compressor pumping stops, but it is finite. The number and size of reservoirs vary with the vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
If you turn off the engine and depress the service brake pedal repeatedly, air pressure should not leak down more than:
- A. 10 psi per minute
- B. 3 psi per minute with brakes applied (3 psi for single vehicles, 4 psi for combination)
- C. Any amount if brakes still feel firm
- D. 1 psi per minute
Answer and explanation
Correct answer: 3 psi per minute with brakes applied (3 psi for single vehicles, 4 psi for combination)
FMCSA pre-trip air brake test: with engine off and brakes applied, pressure should not drop more than 3 psi per minute for a single vehicle, or 4 psi per minute for a combination vehicle. Greater leakage indicates significant air leaks that must be found and repaired.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the function of the brake chambers?
- A. Control the S-cam rotation directly
- B. Monitor air pressure levels
- C. Store compressed air
- D. Convert air pressure into mechanical force that applies the brakes via the pushrod
Answer and explanation
Correct answer: Convert air pressure into mechanical force that applies the brakes via the pushrod
Brake chambers (also called air brake actuators) convert air pressure into mechanical force. When air enters the chamber, it pushes a diaphragm against a plate, moving the pushrod outward, which rotates the slack adjuster and S-cam to apply the brake shoes.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What are 'piggyback' or 'spring brake' chambers?
- A. Additional chambers for trailer tandems only
- B. Double-acting hydraulic cylinders
- C. Spare brake chambers carried on the vehicle
- D. A combination unit where the service brake chamber and spring brake chamber are combined in one housing
Answer and explanation
Correct answer: A combination unit where the service brake chamber and spring brake chamber are combined in one housing
Spring brake chambers (piggyback chambers) combine the service brake chamber and spring brake (emergency/parking brake) in one unit. The front portion operates service brakes; the rear portion contains a powerful spring for emergency and parking braking. Both halves must be inspected.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A 'caging bolt' or 'caging procedure' for spring brakes is used when:
- A. To test brake chamber integrity
- B. Parking on a downgrade
- C. To mechanically compress and hold the spring brake in the released position for maintenance or towing — should only be done by trained technicians
- D. To increase braking power
Answer and explanation
Correct answer: To mechanically compress and hold the spring brake in the released position for maintenance or towing — should only be done by trained technicians
Caging a spring brake mechanically compresses the large internal spring, releasing the brakes for maintenance or when the vehicle must be moved with low air pressure. This is a maintenance procedure requiring training. Improperly caged spring brakes can release suddenly and cause serious injury.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
During pre-trip, you should check that the brake shoe lining is NOT:
- A. Dark in color
- B. Slightly lighter than the drum surface
- C. Made of composite material
- D. Contaminated with oil or grease, cracked, or worn to less than minimum thickness
Answer and explanation
Correct answer: Contaminated with oil or grease, cracked, or worn to less than minimum thickness
Brake lining contaminated with oil or grease will not generate adequate friction, severely reducing braking effectiveness. Cracked linings can break apart during braking. Either condition is an out-of-service violation requiring immediate repair.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is checked during the 'applied leakage test' of an air brake pre-trip inspection?
- A. Whether lights work when brakes are applied
- B. Whether the brake pedal travel is consistent
- C. Whether spring brakes hold on a grade
- D. Whether air leaks from the system when the brakes are held in the applied position — should not drop more than 3 psi per minute
Answer and explanation
Correct answer: Whether air leaks from the system when the brakes are held in the applied position — should not drop more than 3 psi per minute
The applied leakage test: with air pressure built up and engine off, apply and hold the brake pedal. Listen for leaks and watch the pressure gauge. For single vehicles, pressure should not drop more than 3 psi per minute. Excessive leakage indicates brake system faults.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
In Florida's warm, humid climate, why is air dryer maintenance especially important?
- A. Florida's heat evaporates moisture naturally
- B. Florida's high humidity causes more moisture to enter the air system than in dry climates — regular dryer servicing prevents brake line corrosion and keeps the system free of water
- C. Humidity is not a concern for air brakes in warm climates
- D. Air dryers are only needed in northern states
Answer and explanation
Correct answer: Florida's high humidity causes more moisture to enter the air system than in dry climates — regular dryer servicing prevents brake line corrosion and keeps the system free of water
Florida's subtropical climate means air compressors draw in hot, moisture-laden air constantly. This produces more water in the air system than dry-climate states. Regular air dryer maintenance and tank draining are critical in Florida to prevent corrosion and water-induced issues in brake lines and chambers.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When inspecting brake hoses on a trailer, you should specifically look for:
- A. Only the color of the hose fittings
- B. Kinks, cracks, wear from rubbing on frame or suspension components, and secure connections at both ends
- C. Hose color and brand markings
- D. Whether the hose is the correct length
Answer and explanation
Correct answer: Kinks, cracks, wear from rubbing on frame or suspension components, and secure connections at both ends
Trailer brake hoses are exposed to constant flexing, vibration, and potential rubbing on frame components. Check for kinks (restrict air flow), cracks, abrasion wear, and secure connections at both the trailer and glad hands. A failed trailer brake hose can cause sudden loss of trailer braking.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What are 'glad hands'?
- A. Coupling devices on hoses that connect air lines between the tractor and trailer
- B. The handles on trailer landing gear
- C. The hand rails on the tractor
- D. Special gloves worn by CDL drivers
Answer and explanation
Correct answer: Coupling devices on hoses that connect air lines between the tractor and trailer
Glad hands are the coupling connectors on the ends of tractor and trailer air hoses. They snap together to connect service and emergency air lines between tractor and trailer. Check that they are properly connected and sealed — air leaks at glad hands can cause brake issues.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The service line (blue glad hand) connects to the trailer service brake system. The emergency line (red glad hand) connects to:
- A. The trailer lights only
- B. The trailer spring brake system — if this line loses air pressure, trailer spring brakes automatically apply
- C. The trailer fuel system
- D. The trailer refrigeration unit
Answer and explanation
Correct answer: The trailer spring brake system — if this line loses air pressure, trailer spring brakes automatically apply
The emergency line (red) supplies air to the trailer's spring brake relay valve. If this line is disconnected or loses pressure, the trailer spring brakes apply automatically — this is the emergency brake function. The service line (blue) carries air for trailer service brake applications.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
'Air brake lag time' refers to:
- A. The time it takes for air to refill the tanks
- B. The slight delay (about 0.4 seconds) between pressing the brake pedal and the brakes actually applying
- C. The time for brakes to cool after use
- D. The delay before the low air warning activates
Answer and explanation
Correct answer: The slight delay (about 0.4 seconds) between pressing the brake pedal and the brakes actually applying
Air brake lag is an inherent characteristic: it takes about 0.4 seconds for air to travel from the foot valve through the lines to the brake chambers. At 55 mph, the vehicle travels about 32 feet during this lag. This must be factored into following distance calculations.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the 'relay valve' in a trailer air brake system?
- A. A valve that controls the trailer lighting system
- B. A valve that connects the tractor and trailer emergency systems
- C. The same as the service valve on the tractor
- D. A valve that speeds up brake application and release at the trailer by using local air from the trailer reservoir
Answer and explanation
Correct answer: A valve that speeds up brake application and release at the trailer by using local air from the trailer reservoir
Relay valves improve brake response time on long trailers by using air from a trailer-mounted reservoir, controlled by the air signal from the tractor. Without relay valves, air would have to travel from the tractor all the way to each trailer axle brake chamber, creating dangerous lag time.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
If you cannot move your vehicle due to spring brakes being fully applied with low air pressure, you should:
- A. Force the vehicle to move by accelerating hard
- B. Call for a tow immediately regardless of air pressure
- C. Cage the spring brakes yourself and drive to a shop
- D. Not attempt to drive — rebuild air pressure to normal operating level (around 100+ psi) before moving
Answer and explanation
Correct answer: Not attempt to drive — rebuild air pressure to normal operating level (around 100+ psi) before moving
If spring brakes are applied due to low air pressure, do not try to drive the vehicle. Rebuild air pressure to normal operating levels first. Spring brakes are designed to hold the vehicle stationary — driving against them can damage the drivetrain and brake components.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Brake fade is more likely to occur when:
- A. When driving at low speeds
- B. In cold weather only
- C. On long steep downgrades when brakes are continuously applied and overheat
- D. Driving on flat terrain
Answer and explanation
Correct answer: On long steep downgrades when brakes are continuously applied and overheat
Brake fade occurs from overheating caused by continuous application on long downgrades. As brake drums overheat (above 400°F), the friction coefficient drops and braking effectiveness decreases dramatically. Prevent fade by selecting the correct low gear and using snub braking technique.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What should you do if you hear a continuous air leak during your pre-trip inspection?
- A. Only check at the glad hands for the source
- B. Continue the pre-trip and drive if pressure stays above 60 psi
- C. Do NOT drive the vehicle — locate and repair the leak before operating
- D. Report it to dispatch and drive to the shop carefully
Answer and explanation
Correct answer: Do NOT drive the vehicle — locate and repair the leak before operating
A continuous air leak indicates a significant problem that will cause air pressure to drop during operation. If pressure drops to 60 psi, the low air warning activates. Below 20-45 psi, spring brakes apply automatically. Do not operate a vehicle with a significant air leak.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The 'parking brake' control in a CMV cab controls:
- A. The service brakes only
- B. An electric parking brake motor
- C. The transmission park lock
- D. The spring brakes — pulling the yellow diamond knob (tractor) or red octagon knob (trailer) releases air and applies spring brakes
Answer and explanation
Correct answer: The spring brakes — pulling the yellow diamond knob (tractor) or red octagon knob (trailer) releases air and applies spring brakes
The parking brake control (yellow diamond-shaped knob for tractor, red octagonal knob for trailer) controls the spring brakes. Pulling the knob releases air from the spring brake chambers, allowing the powerful internal spring to apply the brakes firmly. Pushing it in allows air to release (overcome) the spring.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the 'modulating control valve' used for?
- A. To control trailer brake application rate only
- B. To apply spring brakes gradually rather than all at once, giving the driver some control during an emergency
- C. To modulate air pressure to the air dryer
- D. To control the rate of air compressor operation
Answer and explanation
Correct answer: To apply spring brakes gradually rather than all at once, giving the driver some control during an emergency
The modulating control valve (spring brake modulating valve) allows the driver to apply the spring brakes gradually in an emergency — for example, if service brakes fail. Without it, the spring brakes apply all at once, which can cause loss of vehicle control.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
You notice a thin coating of oil on the inside of your brake drums during pre-trip. This means:
- A. Normal — oil provides lubrication for brake components
- B. Only a concern if the oil is fresh
- C. Only a concern on wet days
- D. Oil has contaminated the brake linings — braking effectiveness is severely reduced and the vehicle should not be driven until repaired
Answer and explanation
Correct answer: Oil has contaminated the brake linings — braking effectiveness is severely reduced and the vehicle should not be driven until repaired
Oil contamination on brake drums and linings is a serious safety defect. Oil dramatically reduces the friction between the lining and drum, causing brake failure. The vehicle must not be driven until the source of the oil leak is found and repaired, and all contaminated linings are replaced.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
During pre-trip inspection, you should confirm that the tractor protection valve will close (protect the tractor brakes) if:
- A. Only when the hand valve is applied
- B. Only during emergency braking
- C. Only during rainy conditions
- D. Trailer air pressure drops to 20-45 psi — this prevents the tractor from losing its air supply through the trailer system
Answer and explanation
Correct answer: Trailer air pressure drops to 20-45 psi — this prevents the tractor from losing its air supply through the trailer system
The tractor protection valve closes automatically if trailer air pressure drops to 20-45 psi, isolating the tractor from the trailer air system. This prevents a trailer air loss (broken hose, glad hand disconnection) from also depleting the tractor's air supply and disabling tractor brakes.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When driving in Florida's heat, which specific air brake concern should you be most vigilant about?
- A. Brake fade never occurs in warm climates
- B. Brake overheating on Florida's flat roads is minimal — but brake fade from heavy traffic braking on Florida Turnpike grades and bridge ramps can occur
- C. Air compressors are less efficient in heat
- D. Brakes freezing due to low temperatures
Answer and explanation
Correct answer: Brake overheating on Florida's flat roads is minimal — but brake fade from heavy traffic braking on Florida Turnpike grades and bridge ramps can occur
While Florida lacks major mountain grades, brake overheating can occur on bridge approach ramps, in heavy stop-and-go traffic, and on Florida Turnpike grades in South Florida. Additionally, Florida's heat and humidity stress the air system more than in temperate climates. Monitor brake temperature and use proper technique.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the maximum air loss rate per minute for a single vehicle with brakes released?
- A. 5 psi per minute
- B. Any rate is acceptable if pressure stays above 60 psi
- C. 3 psi per minute
- D. 1 psi per minute
Answer and explanation
Correct answer: 1 psi per minute
With engine off, brakes released, pressure should not drop more than 1 psi per minute on a single vehicle (or 2 psi per minute on a combination vehicle). This test identifies leaks in the air supply system. Exceeding these rates indicates significant leakage that must be repaired.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What does a 'quick release valve' do?
- A. Controls the rate of air pressure buildup
- B. Speeds up the release of brakes by allowing air to exhaust locally rather than returning to the brake valve
- C. Releases air from the air dryer
- D. Provides emergency brake release
Answer and explanation
Correct answer: Speeds up the release of brakes by allowing air to exhaust locally rather than returning to the brake valve
Quick release valves are installed near brake chambers to speed brake release. When the brake pedal is released, the quick release valve exhausts air to atmosphere locally rather than sending it back through long lines to the brake valve, improving brake release response time.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the correct way to test the emergency air line by disconnecting it?
- A. Stop the vehicle, charge the trailer air system, then disconnect the emergency line — trailer spring brakes should apply within seconds
- B. Never disconnect the emergency line for any reason
- C. Disconnect it at highway speed to see if trailer brakes apply
- D. This test should only be done by mechanics
Answer and explanation
Correct answer: Stop the vehicle, charge the trailer air system, then disconnect the emergency line — trailer spring brakes should apply within seconds
To test emergency air line function: with air charged and vehicle stopped, disconnect the emergency (red) glad hand. The trailer spring brakes should apply automatically within a few seconds as air escapes. This verifies the emergency system will function if the line breaks during operation.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What does 'wig-wag' or 'low air warning device' do when it activates?
- A. Drops a mechanical arm into the driver's view (or sounds an alarm) warning that air pressure has dropped to 60 psi
- B. Controls trailer emergency brakes
- C. Activates the parking brake automatically
- D. Signals a turn
Answer and explanation
Correct answer: Drops a mechanical arm into the driver's view (or sounds an alarm) warning that air pressure has dropped to 60 psi
A wig-wag device drops a mechanical arm into the driver's line of sight to warn of dangerously low air pressure (60 psi). Modern vehicles use warning lights and buzzers. When the wig-wag drops, you must find a safe place to stop before air pressure drops further.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why must you NOT release the brakes by pushing in the spring brake control valve (yellow knob) while moving if the spring brakes are the only thing stopping the vehicle?
- A. The service brakes have failed — releasing spring brakes without functional service brakes leaves you with no braking ability while moving
- B. It can cause jackknifing
- C. It is prohibited by Florida law only
- D. It drains the air tanks
Answer and explanation
Correct answer: The service brakes have failed — releasing spring brakes without functional service brakes leaves you with no braking ability while moving
If service brakes have failed and only spring brakes are holding the vehicle, do not release the spring brake control. You have no functional service brakes. Stay stopped and call for assistance. Releasing the spring brakes removes your only braking ability.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the function of an automatic slack adjuster?
- A. To automatically maintain correct brake adjustment as lining wears — eliminating the need for manual adjustment
- B. To control the rate of brake application
- C. To automatically drain water from air tanks
- D. To automatically adjust air pressure based on speed
Answer and explanation
Correct answer: To automatically maintain correct brake adjustment as lining wears — eliminating the need for manual adjustment
Automatic slack adjusters (ASA) continuously maintain correct brake shoe-to-drum clearance as linings wear. Vehicles equipped with ASAs should still have pushrod stroke checked during pre-trip inspection — a stiff or stuck ASA can result in an out-of-adjustment brake even with an automatic system.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the minimum air pressure required before driving a CMV with air brakes?
- A. 100 psi — allow the compressor to build to at least 100 psi before moving
- B. 60 psi
- C. 30 psi
- D. Any pressure above 20 psi
Answer and explanation
Correct answer: 100 psi — allow the compressor to build to at least 100 psi before moving
The air brake system must be charged to at least 100 psi before moving. Below 60 psi, the low air warning is active. Below 20-45 psi, spring brakes apply automatically. Building pressure to at least 100 psi before driving ensures you have adequate reserve for safe operation.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is a 'two-way check valve'?
- A. A switch to alternate between service and emergency brakes
- B. A one-way valve that allows air flow from whichever tank has the highest pressure to the delivery side, while preventing backflow
- C. A valve tested during pre-trip by pressing twice
- D. A valve that checks fluid levels automatically
Answer and explanation
Correct answer: A one-way valve that allows air flow from whichever tank has the highest pressure to the delivery side, while preventing backflow
A two-way check valve (or double check valve) allows air from either of two supply circuits to pass through to the delivery circuit, while preventing air from flowing backward from one supply to the other. This is used to ensure dual air systems can supply each other's brake circuits.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The color of the parking brake control knob on a tractor is:
- A. Yellow (diamond-shaped)
- B. Blue
- C. Red
- D. Green
Answer and explanation
Correct answer: Yellow (diamond-shaped)
The tractor parking brake control is a yellow, diamond-shaped knob. The trailer parking brake control is a red, octagonal knob. These shapes and colors are standardized so drivers can identify them without looking. Yellow controls the tractor spring brakes; red controls the trailer.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What should you do if the low air warning light comes on while you are driving?
- A. Increase speed to reach the next exit faster
- B. Continue to the nearest truck stop
- C. Only stop if the buzzer also sounds
- D. Pull off the road safely immediately — you may have very little air remaining to stop
Answer and explanation
Correct answer: Pull off the road safely immediately — you may have very little air remaining to stop
When the low air warning activates (60 psi), you may have only enough air for a few more brake applications before pressure drops to spring brake application level. Pull off the road safely and immediately. Do not continue driving.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Air brake systems are required on CMVs over what weight?
- A. Only on combination vehicles over 80,000 lbs
- B. Only on Class A vehicles
- C. 16,000 lbs GVWR
- D. Air brakes are required on CMVs where the manufacturer has determined air brakes are needed for safe stopping — generally 10,000 lbs+ GVW combinations or heavy single vehicles
Answer and explanation
Correct answer: Air brakes are required on CMVs where the manufacturer has determined air brakes are needed for safe stopping — generally 10,000 lbs+ GVW combinations or heavy single vehicles
There is no single universal weight threshold — air brakes are required based on manufacturer design and federal safety standards for vehicles where hydraulic brakes cannot provide adequate stopping performance. Most CMVs over 10,000 lbs use air brakes, and virtually all Class A and B CDL vehicles are air brake equipped.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What indicates a properly functioning air compressor during pre-trip?
- A. The safety valve opens during normal operation
- B. Pressure builds from 85 to 100 psi within 45 seconds with the engine running
- C. The compressor runs continuously
- D. Pressure never builds above 80 psi
Answer and explanation
Correct answer: Pressure builds from 85 to 100 psi within 45 seconds with the engine running
A healthy compressor builds pressure from 85 to 100 psi within 45 seconds at normal engine RPM. Slower build-up time indicates the compressor may be worn or there is a significant air leak. The governor should also cycle properly (cut-in ~100 psi, cut-out ~125 psi).
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
On flat roads in Florida with frequent stop-and-go traffic, a key air brake concern is:
- A. Air system freezing
- B. No special concerns in flat terrain
- C. Spring brake failure
- D. Brake overheating from frequent full-stop traffic — especially on heavy vehicles in Miami, Tampa, and Orlando traffic corridors
Answer and explanation
Correct answer: Brake overheating from frequent full-stop traffic — especially on heavy vehicles in Miami, Tampa, and Orlando traffic corridors
Florida's congested urban corridors (I-95 Miami, I-4 Orlando, I-275 Tampa) subject CMV air brakes to frequent hard stops in heavy traffic. This generates significant brake heat on flat terrain. Monitor brake temperature and use engine braking when possible to reduce brake wear and heat in stop-and-go conditions.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is 'total stopping distance' for a vehicle with air brakes?
- A. Braking distance only
- B. Perception distance + reaction distance + air brake lag distance + braking distance
- C. The distance from when you see a hazard to when wheels lock
- D. Only the distance needed to apply the brakes
Answer and explanation
Correct answer: Perception distance + reaction distance + air brake lag distance + braking distance
Total stopping distance has four components: 1) Perception distance (time to recognize hazard), 2) Reaction distance (foot moves to brake pedal), 3) Air brake lag (0.4 sec for air to reach chambers), 4) Braking distance (vehicle slows and stops). All four add up to significantly more than most drivers realize.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why should you never 'squeeze' or hold the trailer brake hand valve while driving?
- A. The hand valve is not designed for driving
- B. Using only the trailer brakes while moving can cause the trailer to jackknife — the trailer wheels lock while tractor wheels keep moving
- C. Florida law prohibits it
- D. It activates the spring brakes inadvertently
Answer and explanation
Correct answer: Using only the trailer brakes while moving can cause the trailer to jackknife — the trailer wheels lock while tractor wheels keep moving
Applying only the trailer brakes (via hand valve) without applying tractor brakes can cause jackknifing — the trailer decelerates while the tractor continues pushing, causing the trailer to swing sideways. The service brake pedal applies all brakes simultaneously for safe, controlled stops.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What should you hear when you test the service brakes during pre-trip inspection?
- A. A slight hissing is normal and acceptable
- B. NO air leaks from the chambers or lines when holding the brakes applied, and the pedal should not drift slowly to the floor
- C. The brakes are self-testing — no test is needed
- D. A loud bang indicating full brake engagement
Answer and explanation
Correct answer: NO air leaks from the chambers or lines when holding the brakes applied, and the pedal should not drift slowly to the floor
During the service brake test with engine off: apply and hold the brakes, listen for air leaks at chamber connections and hoses, and watch the gauges. No excessive leakage should occur (max 3 psi/min single, 4 psi/min combination), and the pedal should hold firm without slow drift.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What approximate cut-out pressure does the CDL manual give as an example, subject to manufacturer specifications?
- A. 150 psi
- B. 100 psi
- C. 60 psi
- D. 125 psi
Answer and explanation
Correct answer: 125 psi
The manual uses approximately 100 psi for cut-in and 125 psi for cut-out. These are examples, not universal limits for every system. Check the manufacturer's specifications when testing the actual vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What approximate cut-in pressure does the CDL manual give as an example, subject to manufacturer specifications?
- A. 60 psi
- B. 125 psi
- C. 80 psi
- D. 100 psi
Answer and explanation
Correct answer: 100 psi
The manual uses approximately 100 psi for cut-in and 125 psi for cut-out. These are examples, not universal limits for every system. Check the manufacturer's specifications when testing the actual vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When testing service brakes at low speed before beginning a trip, you should:
- A. Push the brake pedal firmly at about 5 mph and feel for pulling, unusual sounds, or delays — any abnormality should be investigated before continuing
- B. Apply maximum pressure to test effectiveness
- C. Test is only required weekly, not daily
- D. Test only on an open highway
Answer and explanation
Correct answer: Push the brake pedal firmly at about 5 mph and feel for pulling, unusual sounds, or delays — any abnormality should be investigated before continuing
Always perform a low-speed service brake test before starting a trip. At about 5 mph, apply the brakes firmly and feel for: pulling to one side, unusual grinding or squealing, delayed response, or soft pedal. These indicate brake problems that must be investigated before driving further.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why does warm weather not eliminate the need for proper air-tank drainage?
- A. Daily draining is not needed if the vehicle has an air dryer
- B. Florida does not require daily draining
- C. Water and oil can accumulate even in warm weather; drain manual tanks as prescribed and check automatic drains
- D. Only weekly draining is required in warm climates
Answer and explanation
Correct answer: Water and oil can accumulate even in warm weather; drain manual tanks as prescribed and check automatic drains
For tanks with manual drains, remove the accumulated water and oil at the end of each working day. Automatic draining is a different arrangement and must be checked for the fitted equipment. Warm weather does not eliminate moisture contamination; do not infer a unique state-specific draining rule.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What does a 'dead end' air line connector (dummy coupling) do?
- A. Controls air pressure during coupling
- B. Prevents quick release valve activation
- C. Caps open air lines to prevent contamination and air loss when a trailer is not connected
- D. Seals the emergency line permanently
Answer and explanation
Correct answer: Caps open air lines to prevent contamination and air loss when a trailer is not connected
Dummy couplings (also called dummy connectors or glad hand seals) cap open air line ends (glad hands) to prevent contamination and dirt from entering the air system when a trailer is not attached. They also prevent small air leaks from open connections.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
During the pre-trip air brake inspection, after building pressure to cut-out, you should turn off the engine and then:
- A. Start the vehicle again immediately
- B. Wait and watch the gauges — no brake application — to check for static (no-brake) air loss
- C. Drive the vehicle slowly to test pressure
- D. Immediately apply the brakes hard
Answer and explanation
Correct answer: Wait and watch the gauges — no brake application — to check for static (no-brake) air loss
After building to cut-out with engine off, first check STATIC leakage (no brakes applied) — should not exceed 1 psi/minute (single) or 2 psi/minute (combination). Then test APPLIED leakage by holding brakes down — should not exceed 3 psi/minute (single) or 4 psi/minute (combination).
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A properly adjusted brake has a pushrod stroke of:
- A. Within the maximum stroke limit for that chamber size — typically under 2 inches depending on chamber type
- B. Any length as long as the brakes feel firm
- C. Must be set by a certified mechanic only — cannot be verified during pre-trip
- D. Exactly 3 inches for all vehicles
Answer and explanation
Correct answer: Within the maximum stroke limit for that chamber size — typically under 2 inches depending on chamber type
Maximum brake adjustment limits vary by brake chamber type and size but generally must be under 2 inches. Check the specific limit for your vehicle's chamber type. Out-of-adjustment brakes (exceeding the limit) are an FMCSA out-of-service condition and significantly reduce braking effectiveness.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What should happen to braking distance if brake adjustment becomes out of spec?
- A. Braking distance decreases
- B. Braking distance increases — sometimes dramatically
- C. No change to braking distance
- D. Only affects emergency stops, not normal braking
Answer and explanation
Correct answer: Braking distance increases — sometimes dramatically
Out-of-adjustment brakes mean less brake shoe-to-drum contact force for a given air pressure application. Less contact force means less friction and less braking torque, requiring more distance to stop. Severely out-of-adjustment brakes can increase stopping distance by 50% or more.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why are spring brakes NOT used for normal parking on grades in Florida?
- A. They are too difficult to release
- B. Spring brakes are only for emergencies
- C. Florida law prohibits spring brake use for parking
- D. Spring brakes alone may not hold a loaded vehicle on a steep grade — wheel chocks must always be used
Answer and explanation
Correct answer: Spring brakes alone may not hold a loaded vehicle on a steep grade — wheel chocks must always be used
Spring brakes provide substantial holding force but may not be sufficient alone on steep grades with a loaded vehicle. Always chock the wheels when parking on any grade, and use spring brakes in combination with wheel chocks for maximum safety.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the main advantage of air brakes over hydraulic brakes on heavy vehicles?
- A. Air brake systems can use storage tanks for reserve capacity and fail-safe spring brakes — hydraulic systems can fail completely if fluid is lost
- B. Air brake pedals require less effort
- C. Air is cheaper than hydraulic fluid
- D. Air brakes stop the vehicle faster in all conditions
Answer and explanation
Correct answer: Air brake systems can use storage tanks for reserve capacity and fail-safe spring brakes — hydraulic systems can fail completely if fluid is lost
Air brake systems have built-in safety advantages: stored air in tanks provides reserve braking capacity even if the compressor fails, and spring brakes automatically apply if air pressure drops too low. Hydraulic brakes have no such fail-safe — total fluid loss means total brake loss.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What is the purpose of the 'inversion valve' in some air brake systems?
- A. It reduces air pressure to front brakes to prevent front wheel lockup
- B. It inverts air pressure readings
- C. It reverses spring brake application
- D. It controls air flow between circuits
Answer and explanation
Correct answer: It reduces air pressure to front brakes to prevent front wheel lockup
A ratio valve (inversion valve) reduces air pressure delivered to front brake chambers, preventing front wheel lockup during hard stops. Locked front wheels eliminate steering. The valve ensures rear brakes do more work while maintaining front wheel steering control.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
An air dryer must be serviced periodically because:
- A. The desiccant becomes saturated with moisture and must be replaced or regenerated
- B. It never needs servicing
- C. Florida law requires annual air dryer replacement
- D. It requires lubrication every 10,000 miles
Answer and explanation
Correct answer: The desiccant becomes saturated with moisture and must be replaced or regenerated
Air dryers use desiccant (silica gel) to absorb moisture from compressed air. Over time the desiccant becomes saturated and loses effectiveness. Service intervals typically range from 1 to 3 years. A failed air dryer results in water buildup in the brake system.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When inspecting brake linings through a wheel opening, you are checking for:
- A. Whether the lining has the correct brand name
- B. Minimum thickness, cracks, oil/grease contamination, and secure attachment to the shoe
- C. Only lining surface texture
- D. Lining color only
Answer and explanation
Correct answer: Minimum thickness, cracks, oil/grease contamination, and secure attachment to the shoe
Brake lining inspection checks: thickness (minimum 1/4 inch non-shoe, 1/8 inch shoe adjacent to drum), cracks, oil or grease contamination, and secure fastening to the shoe. Any of these defects is an out-of-service condition.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When approaching a railroad crossing with air brakes, you should:
- A. Downshift only, no braking needed
- B. Brake hard to stop quickly
- C. Only stop if lights are flashing
- D. Reduce speed and apply brakes smoothly well before the crossing — avoid braking on the tracks
Answer and explanation
Correct answer: Reduce speed and apply brakes smoothly well before the crossing — avoid braking on the tracks
Brake well before the railroad crossing, not on the tracks. The 15-50 foot required stop distance before the nearest rail must be achieved before reaching the tracks. Braking on the tracks risks becoming stuck.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Rapidly fluctuating air pressure gauge readings indicate:
- A. Normal compressor operation
- B. The gauge is reading correctly
- C. Possible governor malfunction or serious air system leak requiring immediate attention
- D. Low fuel causing pressure fluctuation
Answer and explanation
Correct answer: Possible governor malfunction or serious air system leak requiring immediate attention
Rapid pressure gauge fluctuation indicates a possible governor problem, compressor issue, or significant intermittent leak. Stable pressure with normal compressor cycling is expected. Rapid fluctuation requires investigation before continued operation.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Maximum permissible air loss rate per minute on a combination vehicle with brakes applied is:
- A. 2 psi per minute
- B. 10 psi per minute
- C. 4 psi per minute
- D. 1 psi per minute
Answer and explanation
Correct answer: 4 psi per minute
For a combination vehicle with brakes applied, air pressure should not drop more than 4 psi per minute. For a single vehicle, the maximum is 3 psi per minute. These rates are verified during pre-trip inspection per FMCSA regulations.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
On a long downgrade, brakes feel less effective than normal. The correct action is:
- A. Shift to a higher gear
- B. Press harder on the brake pedal
- C. Engage the spring brakes to assist
- D. Pull off the road safely and allow brakes to cool — brake fade has begun
Answer and explanation
Correct answer: Pull off the road safely and allow brakes to cool — brake fade has begun
Reduced brake effectiveness on a downgrade is the first sign of brake fade. Pull off safely and allow brakes to cool. If you continue, effectiveness will decrease further, potentially to zero. Inspect drums for heat before continuing.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
What protects the tractor air supply if a trailer brake line is severed?
- A. The tractor protection valve — it closes automatically to isolate the tractor air system
- B. The glad hands seal automatically
- C. The spring brakes prevent air loss
- D. The trailer relay valve
Answer and explanation
Correct answer: The tractor protection valve — it closes automatically to isolate the tractor air system
The tractor protection valve automatically closes when trailer air pressure drops to 20-45 psi from a broken line or disconnected glad hand. This prevents the tractor's air supply from depleting through the damaged trailer system, preserving tractor braking ability.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Chocking wheels before releasing parking brakes serves what purpose?
- A. It helps seat brake drums properly
- B. Required by Florida law only
- C. Prevents the vehicle from rolling if spring brakes release unexpectedly or fail to hold on a grade
- D. Required only when fully loaded
Answer and explanation
Correct answer: Prevents the vehicle from rolling if spring brakes release unexpectedly or fail to hold on a grade
Chocking wheels provides a mechanical backup if spring brakes fail to hold the vehicle. Always chock wheels before releasing spring brakes on a grade or when performing any work under or around the vehicle. This is standard safety practice for CMV operations.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Typical operating pressure range for a standard air brake system is:
- A. 100-125 psi (between compressor cut-in and cut-out)
- B. 60-90 psi
- C. 150-200 psi
- D. 30-60 psi
Answer and explanation
Correct answer: 100-125 psi (between compressor cut-in and cut-out)
Standard air brake systems operate between about 100 psi (compressor cut-in) and 125 psi (compressor cut-out). Below 60 psi, the low pressure warning activates. Below 20-45 psi, spring brakes apply automatically.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why is it important to check brake hose connections during pre-trip?
- A. To verify the hose brand
- B. Loose connections cause air leaks and reduced braking effectiveness
- C. To measure hose diameter
- D. To check hose age only
Answer and explanation
Correct answer: Loose connections cause air leaks and reduced braking effectiveness
Loose or leaking hose connections reduce air pressure and braking effectiveness. A severe connection leak can rapidly deplete tank pressure, triggering emergency spring brake application. Check all connections for tightness and signs of air leaks.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
At what air pressure must the low air pressure warning signal activate, at the latest?
- A. At or before 60 psi
- B. At or before 40 psi
- C. Only after the spring brakes have already activated
- D. There is no federally required activation point
Answer and explanation
Correct answer: At or before 60 psi
Federal regulation requires the low air pressure warning signal to activate at or before 60 psi in the air tanks, giving the driver a clear alert well before pressure drops low enough to affect braking or trigger spring brake application. This is a specific, regulated threshold, not a vague or manufacturer-discretionary figure, and it activates before the spring brakes engage, not after, so the driver has genuine advance warning.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Emergency stop without ABS: you should use:
- A. Pump the brakes rapidly
- B. Apply and hold 50% brake pressure only
- C. Apply maximum pressure until wheels lock then steer around
- D. Stab braking — hard application then release when wheels near lockup, repeat
Answer and explanation
Correct answer: Stab braking — hard application then release when wheels near lockup, repeat
Emergency stop without ABS: stab braking — press hard until wheels are about to lock, immediately release to let wheels roll and steering return, then press hard again. Repeat rapidly. This stops the vehicle while maintaining some steering control.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Excessive brake dust inside a wheel during pre-trip means:
- A. Inspect closer — excessive dust may indicate heavily worn or recently failed linings. Check lining thickness immediately
- B. Only a concern on drive axles
- C. Florida regulations do not address brake dust
- D. Normal — all brake systems produce some dust
Answer and explanation
Correct answer: Inspect closer — excessive dust may indicate heavily worn or recently failed linings. Check lining thickness immediately
Some brake dust inside wheels is normal, but excessive accumulation — especially fresh dark dust — can indicate rapidly wearing or recently failed brake linings. If you see significant brake dust, inspect the lining thickness and condition before proceeding.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
An automatic drain valve:
- A. Automatically adjusts brake pressure
- B. Automatically releases all air when starting the engine
- C. Automatically expels accumulated water and oil; the operating cycle depends on the fitted drain
- D. Releases all air when the engine is turned off
Answer and explanation
Correct answer: Automatically expels accumulated water and oil; the operating cycle depends on the fitted drain
Automatic drains expel water and oil from tanks; some also permit manual draining. The manual does not prescribe one drain event on every service-brake application or every build-up test. Check operation using the procedure for the fitted drain rather than assuming a universal trigger.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The service (brake application) air line glad hand color is:
- A. Yellow
- B. Green
- C. Red
- D. Blue
Answer and explanation
Correct answer: Blue
The service air line glad hand is blue. The emergency (supply) line is red. This color coding is standardized. Crossing the connections can result in sudden brake application or brake failure.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
If spring brakes are applied due to low air pressure, you should:
- A. Rebuild air pressure to normal operating level before moving
- B. Call for a tow immediately
- C. Cage the spring brakes yourself and drive to a shop
- D. Force the vehicle to move by accelerating hard
Answer and explanation
Correct answer: Rebuild air pressure to normal operating level before moving
If spring brakes are applied due to low air pressure, do not try to drive. Rebuild air pressure to normal operating levels first. Spring brakes are designed to hold the vehicle stationary — driving against them can damage the drivetrain and brake components.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Air brake lag distance at 55 mph adds approximately how many feet before brakes begin applying?
- A. Only matters in wet conditions
- B. Only affects emergency stops
- C. 32 feet — 0.4 seconds of lag at 55 mph
- D. Less than 1 foot
Answer and explanation
Correct answer: 32 feet — 0.4 seconds of lag at 55 mph
Air brake lag (approximately 0.4 seconds) adds about 32 feet of travel at 55 mph before brakes begin to slow the vehicle. This must be added to perception, reaction, and braking distances when calculating total stopping distance.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A 'bobtail' tractor (no trailer) has which braking characteristic?
- A. Always easier to stop — less weight
- B. Faster because there is less weight
- C. The same stopping distance as when pulling a loaded trailer
- D. More difficult — rear drive axles carry little weight and lock easily under braking, potentially causing jackknife
Answer and explanation
Correct answer: More difficult — rear drive axles carry little weight and lock easily under braking, potentially causing jackknife
A bobtail tractor is counterintuitively MORE difficult to stop than with a loaded trailer. The drive axles carry very little weight without a trailer load, so they lock easily under braking. Brake very gently when bobtailing.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
To verify spring brakes hold during pre-trip:
- A. Cannot be tested during pre-trip
- B. Test only on flat ground
- C. Only verify the knob is in correct position
- D. On a slight grade with parking brakes applied, gently attempt to move the vehicle — they must hold
Answer and explanation
Correct answer: On a slight grade with parking brakes applied, gently attempt to move the vehicle — they must hold
To verify spring brakes hold: with spring brakes applied, attempt to move the vehicle gently. The vehicle should not move. If spring brakes cannot hold the vehicle stationary, they must be repaired before the vehicle is moved.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why do combination vehicles have higher air loss rates than single vehicles?
- A. Air loss rate is the same for all vehicle types
- B. Combination vehicles have more efficient brakes
- C. Single vehicles actually use more air
- D. More brake chambers and longer air lines mean more potential leak points and more air used per application
Answer and explanation
Correct answer: More brake chambers and longer air lines mean more potential leak points and more air used per application
A combination vehicle has more brake chambers, longer air lines, more fittings and connections, and more potential leak points than a single vehicle. This is why the applied leakage test allows 4 psi/minute for combinations versus 3 psi/minute for single vehicles.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Multiple out-of-adjustment slack adjusters found during pre-trip indicates:
- A. Likely systematic maintenance neglect or ASA malfunction — requires full brake inspection before operation
- B. Normal wear pattern
- C. Only the rear axle brakes are affected
- D. Drums need resurfacing only
Answer and explanation
Correct answer: Likely systematic maintenance neglect or ASA malfunction — requires full brake inspection before operation
Multiple out-of-adjustment slack adjusters indicate systemic maintenance problems. Automatic slack adjusters should maintain proper adjustment automatically. Multiple failures suggest the ASAs are malfunctioning or the brake system has been neglected. A full brake inspection is required.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A double check valve allows:
- A. A brake chamber with dual diaphragms
- B. Air from whichever of two supply circuits has higher pressure to reach the delivery side, while preventing backflow
- C. A valve checked twice during pre-trip
- D. A redundant air pressure gauge
Answer and explanation
Correct answer: Air from whichever of two supply circuits has higher pressure to reach the delivery side, while preventing backflow
Double check valves allow air from the primary OR secondary circuit (whichever has higher pressure) to supply the trailer brake system, while preventing backflow. This ensures trailer brakes receive air even if one tractor circuit has a problem.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
If the primary circuit of a dual air system fails while driving, you should:
- A. Switch to secondary circuit using a dashboard control
- B. Apply only the trailer brakes
- C. Pull over safely — braking is reduced with only one circuit operating
- D. Continue — the warning light doesn't require stopping
Answer and explanation
Correct answer: Pull over safely — braking is reduced with only one circuit operating
If one circuit of a dual air system fails (warning light activates), your total braking capacity is reduced. Pull over safely. With only one circuit working, stopping distance is significantly increased and the remaining circuit must handle all braking.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
When the tractor emergency line loses pressure, the trailer emergency relay valve:
- A. Applies the trailer spring brakes automatically by dumping air from the spring brake circuit
- B. Prevents air from leaving the trailer system
- C. Signals the tractor to stop
- D. Connects the service line to the emergency circuit
Answer and explanation
Correct answer: Applies the trailer spring brakes automatically by dumping air from the spring brake circuit
When the tractor emergency line loses pressure, the trailer's emergency relay valve dumps air from the spring brake circuit, causing the trailer spring brakes to apply automatically. This is a critical safety feature that stops runaway trailers.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Extremely hot brake drums after a trip indicate:
- A. Drums are self-lubricating at high temperatures
- B. Normal operation after heavy braking
- C. Only a concern in cold weather
- D. The brakes have been dragging or overused — extremely hot drums can crack and brake fade may have occurred
Answer and explanation
Correct answer: The brakes have been dragging or overused — extremely hot drums can crack and brake fade may have occurred
Extremely hot brake drums indicate overuse (brake dragging or continuous application on a downgrade). Overheated drums can crack, warp, or experience reduced effectiveness. After inspection, identify the cause and allow brakes to cool before continuing.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A 'pressure protection valve' in the air system:
- A. Protects spring brakes from over-compression
- B. Protects air tanks from overpressure
- C. Protects the compressor from overheating
- D. Closes off auxiliary systems (air seats, air horns) when brake system pressure falls too low, protecting brake air supply
Answer and explanation
Correct answer: Closes off auxiliary systems (air seats, air horns) when brake system pressure falls too low, protecting brake air supply
Pressure protection valves close off non-essential pneumatic accessories when service brake air pressure drops below a threshold. This protects the brake system's air supply from being depleted by accessories during low-pressure situations.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
To test trailer service brakes before a long trip:
- A. Move forward slowly, apply only the hand valve — trailer should decelerate the combination
- B. Apply maximum brake pressure at highway speed
- C. Only check that air pressure reaches the trailer
- D. No testing required if vehicle passed inspection
Answer and explanation
Correct answer: Move forward slowly, apply only the hand valve — trailer should decelerate the combination
To test trailer service brakes: build air pressure, release parking brakes, move forward slowly, apply only the hand valve. You should feel the trailer brakes apply and decelerate the combination. If you feel no braking effect, trailer brake lines may be disconnected, crossed, or malfunctioning.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
FMCSA 'mechanically defective' brake criteria includes:
- A. Any brake that has been repaired
- B. Brakes with cracked or broken components, missing hardware, or parts that prevent proper function
- C. Only missing cotter pins
- D. Only drum cracks
Answer and explanation
Correct answer: Brakes with cracked or broken components, missing hardware, or parts that prevent proper function
Mechanically defective brakes under FMCSA out-of-service criteria include: cracked drums, broken springs or components, missing or non-functioning hardware, seized or stuck brake chambers, and any condition that prevents the brake from applying or releasing properly.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Hissing at the foot valve during brake application indicates:
- A. Normal air exhaust
- B. A seal failure in the foot valve allowing air to leak — the foot valve may need replacement
- C. Only an air dryer exhaust
- D. Spring brakes partially engaged
Answer and explanation
Correct answer: A seal failure in the foot valve allowing air to leak — the foot valve may need replacement
Air leaking at the foot valve during brake application indicates an internal seal failure. This leak wastes air and can reduce braking effectiveness. The foot valve needs inspection and likely replacement. Do not operate the vehicle with a significantly leaking foot valve.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
'Running out of air' in an air brake system means:
- A. The vehicle needs refueling
- B. The air conditioning system has failed
- C. Air pressure has dropped below safe levels — spring brakes apply automatically as a fail-safe
- D. The air compressor ran out of oil
Answer and explanation
Correct answer: Air pressure has dropped below safe levels — spring brakes apply automatically as a fail-safe
'Running out of air' means brake system air pressure has dropped below safe levels. Below 60 psi, the low air warning activates. Below 20-45 psi, spring brakes apply automatically. This is one of the most dangerous situations a CMV driver can face.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Abnormally long brake pedal travel before brakes apply is most likely caused by:
- A. New brake pads breaking in
- B. Low air pressure, significantly out-of-adjustment brakes, or a failed component — requires immediate inspection
- C. Florida humidity affecting brake components
- D. Normal variation in brake feel
Answer and explanation
Correct answer: Low air pressure, significantly out-of-adjustment brakes, or a failed component — requires immediate inspection
Abnormally long brake pedal travel indicates low air pressure (check gauge immediately), severely out-of-adjustment brakes, or a failed component. This condition means reduced braking effectiveness. Do not continue driving — find a safe place to stop and investigate.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
'Brake fade' specifically describes:
- A. A visual fade on brake drum finish
- B. The reduction in braking effectiveness caused by heat buildup in brake components during prolonged or heavy use
- C. Brake fluid evaporation
- D. The normal reduction in brake response when ABS activates
Answer and explanation
Correct answer: The reduction in braking effectiveness caused by heat buildup in brake components during prolonged or heavy use
Brake fade is the reduction in braking effectiveness caused by heat. When brake drums overheat, the friction coefficient between drum and lining decreases. In severe cases, effectiveness can drop to near zero. Prevent with proper gear selection and snub braking on downgrades.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Yellow diamond-shaped control knob in the cab controls:
- A. Fifth wheel slide
- B. Tractor parking (spring) brakes
- C. Engine throttle
- D. Trailer air supply
Answer and explanation
Correct answer: Tractor parking (spring) brakes
Yellow diamond-shaped knob = tractor parking/spring brakes. Pull out to apply spring brakes (park); push in to release. Red octagonal knob = trailer air supply and spring brakes. These shapes and colors are standardized to prevent confusion.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Red octagonal control knob in the cab controls:
- A. Trailer air supply and trailer spring brakes
- B. Service brake booster
- C. Tractor spring brakes
- D. Emergency brake system only
Answer and explanation
Correct answer: Trailer air supply and trailer spring brakes
Red octagonal knob = trailer air supply/trailer spring brakes. Pull out to close air supply to trailer (trailer spring brakes apply). Push in to supply air to trailer and release trailer spring brakes. Also used to test trailer spring brakes during pre-trip.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The low pressure warning must activate at or before pressure drops to:
- A. 30 psi
- B. 100 psi
- C. 60 psi — the warning must activate BEFORE pressure drops below 60 psi
- D. 90 psi
Answer and explanation
Correct answer: 60 psi — the warning must activate BEFORE pressure drops below 60 psi
The low air pressure warning must activate at or before air pressure drops to 60 psi. If tested and the warning activates below 60 psi or doesn't activate at all, the warning system is defective — do not operate the vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
After turning off the engine, how long should you wait before checking for leaks?
- A. At least 10 minutes
- B. Wait 5 minutes for accurate reading
- C. Check immediately — no wait required
- D. At least 1 minute to allow pressure to stabilize, then monitor for 1-2 minutes
Answer and explanation
Correct answer: At least 1 minute to allow pressure to stabilize, then monitor for 1-2 minutes
After turning off the engine, wait about 1 minute for the system to stabilize, then monitor pressure for 1-2 minutes. This gives a meaningful measurement of static leakage rate. Checking immediately after shutdown may show normal pressure equalization mistaken for a leak.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Brake imbalance — when brakes apply unequally across the vehicle — is dangerous because:
- A. Different pressure settings between service and parking brakes
- B. When air and spring brakes are both applied simultaneously
- C. When brake drums are different sizes on opposite sides
- D. It causes the vehicle to pull sideways or jackknife during braking
Answer and explanation
Correct answer: It causes the vehicle to pull sideways or jackknife during braking
Brake imbalance occurs when brakes apply with unequal force across the vehicle. This can cause the vehicle to pull strongly to one side (causing loss of control) or, in a combination vehicle, cause jackknifing. Any pulling during braking must be investigated immediately.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
In a cam brake system, which component multiplies pushrod force before applying it to the brake shoes?
- A. Air pressure alone without mechanical advantage
- B. The brake chamber only
- C. The S-cam directly without amplification
- D. The slack adjuster acts as a lever to multiply the pushrod force before applying it to the S-cam
Answer and explanation
Correct answer: The slack adjuster acts as a lever to multiply the pushrod force before applying it to the S-cam
The slack adjuster acts as a lever arm, multiplying the linear force of the pushrod into rotational force on the S-cam shaft. The length of the slack adjuster determines the mechanical advantage. Proper slack adjuster length and condition are critical to brake effectiveness.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
If stopping on railroad tracks is unavoidable, you should:
- A. Abandon the vehicle and run perpendicular to tracks
- B. Honk and hope the train stops
- C. Stay put and wait for the train
- D. Pull forward enough to completely clear the tracks — 15+ feet from the nearest rail
Answer and explanation
Correct answer: Pull forward enough to completely clear the tracks — 15+ feet from the nearest rail
If you must stop near railroad tracks, ensure your vehicle completely clears the tracks — 15+ feet from the nearest rail. If stuck on tracks with a train coming, evacuate immediately and run away at a 45-degree angle in the direction the train is coming from.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A CMV with air brakes should not be moved until air pressure reaches at least:
- A. 50 psi
- B. 150 psi
- C. 100 psi — sufficient for full brake operation with emergency reserve
- D. 60 psi
Answer and explanation
Correct answer: 100 psi — sufficient for full brake operation with emergency reserve
Do not move the vehicle until air pressure is at least 100 psi. This ensures adequate pressure for full brake application and leaves a reserve margin before the 60 psi warning level.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Florida CDL applicants who fail or skip the Air Brakes test receive:
- A. No restriction — a separate air brake test is not required in Florida
- B. A 30-day provisional CDL
- C. An 'L' restriction on their CDL — prohibiting operation of any vehicle with air brakes
- D. A 'K' restriction limiting intrastate operation
Answer and explanation
Correct answer: An 'L' restriction on their CDL — prohibiting operation of any vehicle with air brakes
Failing the Air Brakes knowledge test (or not taking it) results in an L restriction on the CDL. The L restriction prohibits operating any vehicle equipped with air brakes — which includes most Class A tractor-trailers and many Class B vehicles, severely limiting job opportunities.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
'Controlled braking' with air brakes means:
- A. Using the hand valve for all stops
- B. Using only 25% brake pressure at all times
- C. Applying maximum brake pressure without locking wheels — maintaining straight-line control while stopping as quickly as possible
- D. Alternating between brakes and engine braking only
Answer and explanation
Correct answer: Applying maximum brake pressure without locking wheels — maintaining straight-line control while stopping as quickly as possible
Controlled braking means applying firm, steady brake pressure to slow the vehicle without locking wheels. This maintains vehicle control (steering ability) while stopping as efficiently as possible. It is the preferred technique for normal and emergency stops on ABS-equipped vehicles.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Before moving after completing air brake pre-trip tests, you must verify:
- A. Only that the engine is warmed up
- B. That the air dryer has cycled at least once
- C. Air pressure at least 100 psi, all warning lights off, parking brake released, AND wheel chocks removed
- D. Only that the parking brake control is pushed in
Answer and explanation
Correct answer: Air pressure at least 100 psi, all warning lights off, parking brake released, AND wheel chocks removed
Before moving after air brake tests: verify pressure is at least 100 psi, no warning lights are active, parking brake control is pushed in, and most importantly — remove all wheel chocks. Driving with chocks in place can cause wheel damage and loss of control.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
A properly functioning air compressor should rebuild from low pressure to 100 psi at normal operating RPM within:
- A. Less than 30 seconds
- B. A few minutes at most — if it takes more than 3-4 minutes at operating RPM, the compressor needs service
- C. No standard time — depends on weather
- D. At least 1 hour
Answer and explanation
Correct answer: A few minutes at most — if it takes more than 3-4 minutes at operating RPM, the compressor needs service
A properly functioning air compressor should rebuild to operating pressure within a few minutes. The pre-trip test (85-100 psi in 45 seconds) checks compressor capacity at operating RPM. A compressor that cannot meet this standard needs attention.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
The 'static leakage test' (no brakes applied) should show pressure drop of no more than:
- A. 3 psi per minute
- B. 1 psi/minute for single vehicles, 2 psi/minute for combination vehicles
- C. 5 psi per minute
- D. Any rate if pressure stays above 60 psi
Answer and explanation
Correct answer: 1 psi/minute for single vehicles, 2 psi/minute for combination vehicles
Static leakage test: engine off, brakes released, maximum drop is 1 psi/minute (single) or 2 psi/minute (combination). Higher rates indicate supply system leaks that must be repaired before operation.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Why is parking on a grade without chocks dangerous with air brakes?
- A. Nothing — spring brakes are always sufficient
- B. Only if the grade exceeds 15%
- C. If spring brakes fail, there is no backup stopping mechanism — the vehicle can roll freely
- D. Only relevant for unloaded vehicles
Answer and explanation
Correct answer: If spring brakes fail, there is no backup stopping mechanism — the vehicle can roll freely
Spring brakes can fail from worn components, improper adjustment, or mechanical failure. Without chocks, a failed spring brake on a grade has nothing to stop vehicle movement. Always use chocks as a backup to spring brakes when parking on any grade.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
If the red (emergency) glad hand disconnects accidentally while driving, what happens?
- A. The trailer spring brakes apply automatically as the emergency line loses pressure — potentially causing jackknifing at speed
- B. The tractor spring brakes also apply simultaneously
- C. Only the trailer lights turn off
- D. Nothing — glad hands are for parking only
Answer and explanation
Correct answer: The trailer spring brakes apply automatically as the emergency line loses pressure — potentially causing jackknifing at speed
If the red glad hand disconnects while driving, the emergency line loses pressure, triggering automatic trailer spring brake application. At highway speed, sudden trailer deceleration without tractor brakes can cause jackknifing. This is why properly securing glad hand connections during pre-trip is critical.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Before walking away from a parked CMV, the driver should verify:
- A. Spring brakes applied (knob pulled out), wheels chocked on any grade, transmission in gear or park, engine off
- B. Only that the engine is off
- C. That air tanks are fully drained
- D. Only that the lights are off
Answer and explanation
Correct answer: Spring brakes applied (knob pulled out), wheels chocked on any grade, transmission in gear or park, engine off
Before leaving the vehicle: verify spring brakes are applied (yellow knob out for tractor, red knob out for trailer), chock wheels on grades, ensure transmission is in a low gear or park, and turn off the engine. Failure to properly secure a CMV can result in a runaway vehicle.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf
Air brake regulations are:
- A. Federal (49 CFR Part 393) — uniform across all 50 states. A Florida CDL driver in Georgia must comply with the same standards
- B. Only apply in Florida
- C. State standards only for intrastate drivers
- D. Different in Florida than other states
Answer and explanation
Correct answer: Federal (49 CFR Part 393) — uniform across all 50 states. A Florida CDL driver in Georgia must comply with the same standards
Air brake regulations are set by FMCSA (49 CFR Part 393) and are federally mandated — uniform across all 50 states. Florida adopts these standards into state law. All CDL drivers must comply regardless of which state they are operating in.
Source: https://www.flhsmv.gov/pdf/handbooks/englishcdlhandbook.pdf

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Florida Air Brakes CDL Practice Test

What is the purpose of the air compressor in an air brake system?

Study all 125 questions and explanations

What is the purpose of the air compressor in an air brake system?

What does the air compressor governor do?

For manually drained air tanks, how often does the CDL manual call for draining?

What is the purpose of the safety relief valve in an air brake system?

A dual air brake system has two separate air circuits. What is the purpose of this design?

What are the three types of air brake systems?

What should you check first during an air brake pre-trip inspection?

When testing the low air pressure warning during pre-trip, you should:

When should spring brakes automatically come on?

How do you check that spring brakes are working during pre-trip inspection?

What is a 'pushrod stroke' and why is it important to check?

The correct way to brake with an air brake system in normal conditions is:

When driving on a downgrade with air brakes, what is the 'snub braking' technique?

What speed should you set as a target when beginning a downgrade with air brakes?

What happens if you hold the brake pedal down continuously on a long downgrade?

What is the purpose of brake drums?

What does an S-cam do in a drum brake system?

What is the purpose of slack adjusters?

What is a 'wedge brake'?

Disc brakes on commercial vehicles offer which advantage over drum brakes?

During a pre-trip inspection, brake drums should be checked for:

What is the minimum thickness for brake lining before it must be replaced?

What happens when you push the 'Trailer Air Supply' valve in (or pull it out, depending on the vehicle)?

The 'hand valve' or 'trolley valve' in the cab controls:

What is the purpose of air dryers in an air brake system?

During a pre-trip inspection with the key on and engine off, the air pressure should build from 85 to 100 psi within how many seconds?

When checking air hoses and lines during pre-trip inspection, you should look for:

Why should you not 'fan' your brakes (rapidly pump them on and off) in an air brake-equipped vehicle?

What role do air storage tanks play in the air brake system?

If you turn off the engine and depress the service brake pedal repeatedly, air pressure should not leak down more than:

What is the function of the brake chambers?

What are 'piggyback' or 'spring brake' chambers?

A 'caging bolt' or 'caging procedure' for spring brakes is used when:

During pre-trip, you should check that the brake shoe lining is NOT:

What is checked during the 'applied leakage test' of an air brake pre-trip inspection?

In Florida's warm, humid climate, why is air dryer maintenance especially important?

When inspecting brake hoses on a trailer, you should specifically look for:

What are 'glad hands'?

The service line (blue glad hand) connects to the trailer service brake system. The emergency line (red glad hand) connects to:

'Air brake lag time' refers to:

What is the 'relay valve' in a trailer air brake system?

If you cannot move your vehicle due to spring brakes being fully applied with low air pressure, you should:

Brake fade is more likely to occur when:

What should you do if you hear a continuous air leak during your pre-trip inspection?

The 'parking brake' control in a CMV cab controls:

What is the 'modulating control valve' used for?

You notice a thin coating of oil on the inside of your brake drums during pre-trip. This means:

During pre-trip inspection, you should confirm that the tractor protection valve will close (protect the tractor brakes) if:

When driving in Florida's heat, which specific air brake concern should you be most vigilant about?

What is the maximum air loss rate per minute for a single vehicle with brakes released?

What does a 'quick release valve' do?

What is the correct way to test the emergency air line by disconnecting it?

What does 'wig-wag' or 'low air warning device' do when it activates?

Why must you NOT release the brakes by pushing in the spring brake control valve (yellow knob) while moving if the spring brakes are the only thing stopping the vehicle?

What is the function of an automatic slack adjuster?

What is the minimum air pressure required before driving a CMV with air brakes?

What is a 'two-way check valve'?

The color of the parking brake control knob on a tractor is:

What should you do if the low air warning light comes on while you are driving?

Air brake systems are required on CMVs over what weight?

What indicates a properly functioning air compressor during pre-trip?

On flat roads in Florida with frequent stop-and-go traffic, a key air brake concern is:

What is 'total stopping distance' for a vehicle with air brakes?

Why should you never 'squeeze' or hold the trailer brake hand valve while driving?

What should you hear when you test the service brakes during pre-trip inspection?

What approximate cut-out pressure does the CDL manual give as an example, subject to manufacturer specifications?

What approximate cut-in pressure does the CDL manual give as an example, subject to manufacturer specifications?

When testing service brakes at low speed before beginning a trip, you should:

Why does warm weather not eliminate the need for proper air-tank drainage?

What does a 'dead end' air line connector (dummy coupling) do?

During the pre-trip air brake inspection, after building pressure to cut-out, you should turn off the engine and then:

A properly adjusted brake has a pushrod stroke of:

What should happen to braking distance if brake adjustment becomes out of spec?

Why are spring brakes NOT used for normal parking on grades in Florida?

What is the main advantage of air brakes over hydraulic brakes on heavy vehicles?

What is the purpose of the 'inversion valve' in some air brake systems?

An air dryer must be serviced periodically because: