CDL AIR BRAKES EXAM —PRACTICE
QUESTIONS WITH CORRECT ANSWERS
AND RATIONALES
What is the primary purpose of an air brake system on
a commercial motor vehicle?
A. To increase engine horsepower
B. To use compressed air to apply the vehicle's brakes
C. To cool the engine
D. To improve fuel economy
Correct Answer: B. To use compressed air to apply
the vehicle's brakes.
Rationale: Air brake systems use compressed air to
transmit force from the driver's brake control to the
foundation brakes.
,Which component stores compressed air for use by the
braking system?
A. Air compressor
B. Air reservoir
C. Brake chamber
D. Slack adjuster
Correct Answer: B. Air reservoir.
Rationale: Reservoirs, also called tanks, store
compressed air so sufficient pressure is available when
the brakes are applied.
What component supplies compressed air to the air
reservoirs?
A. Air compressor
B. Brake drum
C. S-cam
D. Spring brake
Correct Answer: A. Air compressor.
,Rationale: The compressor takes air from the
atmosphere, compresses it, and sends it to the
reservoirs.
What controls the amount of compressed air supplied
to the reservoirs?
A. Governor
B. Slack adjuster
C. Brake shoe
D. Relay valve
Correct Answer: A. Governor.
Rationale: The governor controls when the compressor
loads and unloads based on system pressure.
What is the purpose of the air compressor governor's
cut-out setting?
A. To release the parking brakes
B. To stop the compressor from pumping air when
pressure reaches the specified maximum
C. To apply the service brakes
, D. To drain the reservoirs
Correct Answer: B. To stop the compressor from
pumping air when pressure reaches the specified
maximum.
Rationale: At cut-out pressure, the governor causes
the compressor to stop building pressure.
What happens when air pressure falls to the
governor's cut-in pressure?
A. The compressor begins pumping air again
B. The brakes automatically lock
C. The engine shuts off
D. The air tanks drain
Correct Answer: A. The compressor begins pumping
air again.
Rationale: Cut-in pressure causes the compressor to
resume building pressure in the reservoirs.
What device removes moisture and oil from
compressed air before it enters the reservoirs?
QUESTIONS WITH CORRECT ANSWERS
AND RATIONALES
What is the primary purpose of an air brake system on
a commercial motor vehicle?
A. To increase engine horsepower
B. To use compressed air to apply the vehicle's brakes
C. To cool the engine
D. To improve fuel economy
Correct Answer: B. To use compressed air to apply
the vehicle's brakes.
Rationale: Air brake systems use compressed air to
transmit force from the driver's brake control to the
foundation brakes.
,Which component stores compressed air for use by the
braking system?
A. Air compressor
B. Air reservoir
C. Brake chamber
D. Slack adjuster
Correct Answer: B. Air reservoir.
Rationale: Reservoirs, also called tanks, store
compressed air so sufficient pressure is available when
the brakes are applied.
What component supplies compressed air to the air
reservoirs?
A. Air compressor
B. Brake drum
C. S-cam
D. Spring brake
Correct Answer: A. Air compressor.
,Rationale: The compressor takes air from the
atmosphere, compresses it, and sends it to the
reservoirs.
What controls the amount of compressed air supplied
to the reservoirs?
A. Governor
B. Slack adjuster
C. Brake shoe
D. Relay valve
Correct Answer: A. Governor.
Rationale: The governor controls when the compressor
loads and unloads based on system pressure.
What is the purpose of the air compressor governor's
cut-out setting?
A. To release the parking brakes
B. To stop the compressor from pumping air when
pressure reaches the specified maximum
C. To apply the service brakes
, D. To drain the reservoirs
Correct Answer: B. To stop the compressor from
pumping air when pressure reaches the specified
maximum.
Rationale: At cut-out pressure, the governor causes
the compressor to stop building pressure.
What happens when air pressure falls to the
governor's cut-in pressure?
A. The compressor begins pumping air again
B. The brakes automatically lock
C. The engine shuts off
D. The air tanks drain
Correct Answer: A. The compressor begins pumping
air again.
Rationale: Cut-in pressure causes the compressor to
resume building pressure in the reservoirs.
What device removes moisture and oil from
compressed air before it enters the reservoirs?
