Exam Practice Questions And Correct

CDL Air Brakes (L) Written Knowledge Exam Practice Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the first thing you should do before driving a vehicle with air

brakes?

• Pump the brakes
• Release the parking brake
• Check the air pressure build-up
• Shift into gear

You must check air pressure to ensure the brakes are functioning before moving the vehicle.

2. The air compressor in an air brake system:

• Is always electric
• Pumps air into the air storage tanks 1 / 4

• Only works when the vehicle is moving
• Uses vacuum to create pressure

The air compressor supplies air to the storage tanks for the brake system.

• What is the purpose of air tanks in an air brake system?
• To store fuel
• To store water for cooling
• To store compressed air for braking
• To measure air pressure

Air tanks store compressed air so brakes can be applied reliably.

• How often should the air compressor governor cut out?
• Every 5 minutes
• When the air pressure reaches the maximum safe level
• When the air pressure reaches the full tank level
• Only during braking

The governor stops the compressor when tanks are full to prevent over-pressurization.

• What is the function of the air dryer in an air brake system?
• Increase air pressure
• Remove moisture and oil from compressed air
• Cool the brakes
• Pump more air into the system 2 / 4

Moisture and oil can damage the brake system; the air dryer prevents this.

• What is the purpose of the supply pressure gauge?
• Measures speed
• Measures engine RPM
• Shows the pressure in the air tanks
• Indicates brake wear

The supply pressure gauge tells the driver how much air is in the storage tanks.

• What is a “low air pressure warning” in an air brake system?
• Light that shows oil is low
• Warning activated when air pressure drops below a safe level
• Alarm for low fuel
• Signal that brakes are too hot

A warning alerts the driver when air pressure falls below the minimum safe level (usually 60 psi).

• What happens when the air pressure drops too low in a vehicle with

air brakes?

• Brakes automatically release
• The spring brakes will engage automatically
• Vehicle speeds up
• Engine shuts down 3 / 4

Spring brakes are designed to apply automatically if air pressure is insufficient.

• How should you test the low-pressure warning signal?
• By pressing the brake pedal hard
• Only when driving
• By slowly reducing air pressure to the warning level
• By turning the engine off

Testing by reducing pressure ensures the warning signal functions correctly.

• What is a parking brake in an air brake system?
• A brake that uses spring pressure to hold the vehicle stationary
• A hand pump
• Foot brake only
• Engine brake

Spring brakes hold the vehicle when air pressure is released.

• How can you release spring brakes in a vehicle?
• Press the accelerator
• Build up air pressure in the system
• Push the brake pedal
• Turn the key off

Spring brakes release when the system is pressurized to prevent them from engaging.

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