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CDL AIR BRAKE PRACTICE TEST EXAM
2026 | 150 QUESTIONS AND CORRECT
DETAILED ANSWERS WITH RATIONALE
|RATED A+ |NEW AND REVISED
1. What is the primary function of a vehicle’s air brake system?
A. To operate the hydraulic brake lines
B. To stop and control the vehicle safely
C. To provide power steering assistance
D. To maintain tire pressure
Rationale: Air brakes are designed to safely stop and control the
vehicle using compressed air.
2. When the air pressure in a typical air brake system drops below 60
psi, what happens?
A. Brakes release automatically
B. The low-pressure warning signal activates
C. The engine shuts down
D. The parking brake disengages
Rationale: Most air brake systems have a warning signal that
activates when pressure falls below a safe level.
3. In a dual air system, the front brakes are controlled by which
system?
A. Both systems together
B. Rear system only
C. Front system only
D. Emergency system only
Rationale: In a dual air system, the front brakes are controlled
independently by the front system.
4. What is the purpose of the air compressor governor?
A. To regulate fuel consumption
B. To control air compressor cut-in and cut-out pressure
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C. To maintain brake lining temperature
D. To release the parking brakes
Rationale: The governor maintains proper air pressure by
turning the compressor on or off at set thresholds.
5. Which type of brake is most common on heavy-duty commercial
vehicles?
A. Hydraulic brakes
B. Electric brakes
C. S-cam air brakes
D. Vacuum brakes
Rationale: S-cam air brakes are standard on most large
commercial vehicles due to reliability and stopping power.
6. A “spring brake” in a commercial vehicle is used primarily for:
A. Service braking during driving
B. Parking and emergency braking
C. Engine braking assistance
D. Trailer brake synchronization
Rationale: Spring brakes automatically apply when air pressure
is lost and are used for parking.
7. During a pre-trip inspection, you notice that the air compressor
leaks continuously. What is the correct action?
A. Tighten the compressor belts only
B. Continue driving at low speed
C. Repair before operating the vehicle
D. Drain the air tanks fully
Rationale: Continuous air leaks can cause brake failure; the
vehicle is unsafe to operate.
8. Which statement about air tanks is true?
A. They should be drained weekly only
B. They are filled with hydraulic fluid
C. Moisture and oil must be drained regularly
D. They automatically self-drain
Rationale: Air tanks collect moisture and oil, which can damage
brake components if not drained.
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9. If a vehicle’s air pressure drops too low while driving, the spring
brakes will:
A. Release gradually
B. Engage slowly
C. Engage fully to stop the vehicle
D. Flash the brake lights
Rationale: Spring brakes are designed to engage automatically in
low-pressure situations for safety.
10. The air brake lag distance is:
A. The distance it takes for the vehicle to accelerate
B. The distance the brakes take to activate after the pedal is
pressed
C. Distance traveled after the emergency brake is applied
D. Not a relevant measurement
Rationale: Air brakes require time for pressurized air to reach
the brake chambers, causing lag.
11. What is the maximum allowable air leak rate for a single-
vehicle system during a one-minute test?
A. 3 psi
B. 5 psi
C. 2 psi
D. 10 psi
Rationale: Federal regulations limit leaks to 2 psi per minute in
single-vehicle systems.
12. How often should air brake systems be inspected for leaks?
A. Monthly
B. Every 10,000 miles
C. Daily as part of pre-trip inspection
D. Weekly
Rationale: Daily inspection ensures air brakes are functioning
safely before operating the vehicle.
13. The “emergency air line” on a trailer is also called:
A. Service line
B. Supply line
C. Return line
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D. Release line
Rationale: The emergency line supplies air to release the trailer’s
spring brakes.
14. What effect does excessive moisture in air tanks have on the
braking system?
A. Increases braking power
B. May freeze in cold weather and block air flow
C. Lubricates the brake components
D. Reduces brake lag distance
Rationale: Moisture can freeze in lines, preventing proper brake
operation.
15. When performing a brake check, you notice one wheel locks
before others. This indicates:
A. Air pressure is too high
B. A brake adjustment or mechanical issue exists
C. The tires are overinflated
D. Nothing abnormal; this is expected
Rationale: Uneven brake application can indicate misadjusted or
damaged brakes.
16. What is the purpose of the slack adjuster?
A. Controls tire inflation
B. Maintains proper brake shoe clearance
C. Adjusts spring brake tension
D. Operates the parking brake
Rationale: Slack adjusters compensate for brake lining wear to
maintain correct brake engagement.
17. The air compressor on a heavy vehicle is usually driven by:
A. Hydraulic pump
B. Battery motor
C. Engine via gears or belts
D. Electric generator
Rationale: Engine-driven compressors supply air while the
vehicle is running.
18. When coupling a trailer, the “glad hands” are used to:
A. Connect electrical power
CDL AIR BRAKE PRACTICE TEST EXAM
2026 | 150 QUESTIONS AND CORRECT
DETAILED ANSWERS WITH RATIONALE
|RATED A+ |NEW AND REVISED
1. What is the primary function of a vehicle’s air brake system?
A. To operate the hydraulic brake lines
B. To stop and control the vehicle safely
C. To provide power steering assistance
D. To maintain tire pressure
Rationale: Air brakes are designed to safely stop and control the
vehicle using compressed air.
2. When the air pressure in a typical air brake system drops below 60
psi, what happens?
A. Brakes release automatically
B. The low-pressure warning signal activates
C. The engine shuts down
D. The parking brake disengages
Rationale: Most air brake systems have a warning signal that
activates when pressure falls below a safe level.
3. In a dual air system, the front brakes are controlled by which
system?
A. Both systems together
B. Rear system only
C. Front system only
D. Emergency system only
Rationale: In a dual air system, the front brakes are controlled
independently by the front system.
4. What is the purpose of the air compressor governor?
A. To regulate fuel consumption
B. To control air compressor cut-in and cut-out pressure
,2|Page
C. To maintain brake lining temperature
D. To release the parking brakes
Rationale: The governor maintains proper air pressure by
turning the compressor on or off at set thresholds.
5. Which type of brake is most common on heavy-duty commercial
vehicles?
A. Hydraulic brakes
B. Electric brakes
C. S-cam air brakes
D. Vacuum brakes
Rationale: S-cam air brakes are standard on most large
commercial vehicles due to reliability and stopping power.
6. A “spring brake” in a commercial vehicle is used primarily for:
A. Service braking during driving
B. Parking and emergency braking
C. Engine braking assistance
D. Trailer brake synchronization
Rationale: Spring brakes automatically apply when air pressure
is lost and are used for parking.
7. During a pre-trip inspection, you notice that the air compressor
leaks continuously. What is the correct action?
A. Tighten the compressor belts only
B. Continue driving at low speed
C. Repair before operating the vehicle
D. Drain the air tanks fully
Rationale: Continuous air leaks can cause brake failure; the
vehicle is unsafe to operate.
8. Which statement about air tanks is true?
A. They should be drained weekly only
B. They are filled with hydraulic fluid
C. Moisture and oil must be drained regularly
D. They automatically self-drain
Rationale: Air tanks collect moisture and oil, which can damage
brake components if not drained.
,3|Page
9. If a vehicle’s air pressure drops too low while driving, the spring
brakes will:
A. Release gradually
B. Engage slowly
C. Engage fully to stop the vehicle
D. Flash the brake lights
Rationale: Spring brakes are designed to engage automatically in
low-pressure situations for safety.
10. The air brake lag distance is:
A. The distance it takes for the vehicle to accelerate
B. The distance the brakes take to activate after the pedal is
pressed
C. Distance traveled after the emergency brake is applied
D. Not a relevant measurement
Rationale: Air brakes require time for pressurized air to reach
the brake chambers, causing lag.
11. What is the maximum allowable air leak rate for a single-
vehicle system during a one-minute test?
A. 3 psi
B. 5 psi
C. 2 psi
D. 10 psi
Rationale: Federal regulations limit leaks to 2 psi per minute in
single-vehicle systems.
12. How often should air brake systems be inspected for leaks?
A. Monthly
B. Every 10,000 miles
C. Daily as part of pre-trip inspection
D. Weekly
Rationale: Daily inspection ensures air brakes are functioning
safely before operating the vehicle.
13. The “emergency air line” on a trailer is also called:
A. Service line
B. Supply line
C. Return line
, 4|Page
D. Release line
Rationale: The emergency line supplies air to release the trailer’s
spring brakes.
14. What effect does excessive moisture in air tanks have on the
braking system?
A. Increases braking power
B. May freeze in cold weather and block air flow
C. Lubricates the brake components
D. Reduces brake lag distance
Rationale: Moisture can freeze in lines, preventing proper brake
operation.
15. When performing a brake check, you notice one wheel locks
before others. This indicates:
A. Air pressure is too high
B. A brake adjustment or mechanical issue exists
C. The tires are overinflated
D. Nothing abnormal; this is expected
Rationale: Uneven brake application can indicate misadjusted or
damaged brakes.
16. What is the purpose of the slack adjuster?
A. Controls tire inflation
B. Maintains proper brake shoe clearance
C. Adjusts spring brake tension
D. Operates the parking brake
Rationale: Slack adjusters compensate for brake lining wear to
maintain correct brake engagement.
17. The air compressor on a heavy vehicle is usually driven by:
A. Hydraulic pump
B. Battery motor
C. Engine via gears or belts
D. Electric generator
Rationale: Engine-driven compressors supply air while the
vehicle is running.
18. When coupling a trailer, the “glad hands” are used to:
A. Connect electrical power
