CDL Air Brake System Parts

The air compressor pumps air into the air storage tanks (reservoirs), providing the compressed air needed to operate the braking system.

Study focusUnderstand compressor, governor, reservoir, and air supply behavior.

Common trapConfusing compressor cut-in/cut-out behavior with brake application.

Spring brakes are held back by air pressure. If air pressure is lost, the springs expand and apply the brakes automatically. They serve as both the emergency and parking brakes.

Study focusUnderstand how spring brakes and parking brakes behave when air pressure changes.

Common trapAssuming parking brakes work independently of air pressure behavior.

The governor controls when the air compressor pumps air into the reservoirs (cut-in) and when it stops pumping (cut-out) to maintain proper system pressure.

Study focusUnderstand compressor, governor, reservoir, and air supply behavior.

Common trapConfusing compressor cut-in/cut-out behavior with brake application.

The air compressor governor typically cuts out, meaning it stops pumping air into the tanks, when the system reaches its maximum pressure of about 125-130 psi.

Study focusUnderstand compressor, governor, reservoir, and air supply behavior.

Common trapConfusing compressor cut-in/cut-out behavior with brake application.

Alcohol evaporators put alcohol into the air system to help prevent moisture in the system from freezing in cold weather, which could cause brake failure.

Study focusUnderstand compressor, governor, reservoir, and air supply behavior.

Common trapConfusing compressor cut-in/cut-out behavior with brake application.

Compressed air usually contains water and compressor oil, which can build up in the tanks. If not drained, this mixture can freeze in cold weather and cause brake failure.

Study focusUnderstand compressor, governor, reservoir, and air supply behavior.

Common trapConfusing compressor cut-in/cut-out behavior with brake application.

Air brakes are not instantaneous like hydraulic brakes. Brake lag is the time required for the air to flow through the lines to all the brake chambers (typically about one-half second).

Study focusIdentify key air brake parts and explain how the system produces braking force.

Common trapMemorizing part names without understanding what each part does.

If air pressure falls below 20-45 psi, the spring brakes will engage automatically and bring the vehicle to a rapid stop. You must steer safely off the road before you lose complete mobility.

Study focusUnderstand how spring brakes and parking brakes behave when air pressure changes.

Common trapAssuming parking brakes work independently of air pressure behavior.

Rely primarily on engine braking. When you reach your 'safe speed', apply the brakes firmly until you are 5 mph under that speed, then release them. Repeat this process as needed.

Study focusIdentify key air brake parts and explain how the system produces braking force.

Common trapMemorizing part names without understanding what each part does.

Brake fade is the loss of braking power caused by excessive heat, usually from riding the brakes on a downgrade. Heat causes the drums to expand away from the brake shoes.

Study focusIdentify key air brake parts and explain how the system produces braking force.

Common trapMemorizing part names without understanding what each part does.

The tractor parking brake control is a diamond-shaped, yellow, push-pull control knob.

Study focusUnderstand how spring brakes and parking brakes behave when air pressure changes.

Common trapAssuming parking brakes work independently of air pressure behavior.

Do not use the parking brake if the brakes are extremely hot (they could warp) or if they are very wet in freezing temperatures (they could freeze locked). Use wheel chocks instead.

Study focusUnderstand how spring brakes and parking brakes behave when air pressure changes.

Common trapAssuming parking brakes work independently of air pressure behavior.

ABS only prevents the wheels from locking up during a hard braking event. This allows you to steer around obstacles while stopping, rather than going into an uncontrollable skid.

Study focusIdentify key air brake parts and explain how the system produces braking force.

Common trapMemorizing part names without understanding what each part does.

Slack adjusters are mechanical links between the pushrod and camshaft. They are used to adjust the brakes to ensure the shoes remain close enough to the drums as the friction lining wears away.

Study focusIdentify key air brake parts and explain how the system produces braking force.

Common trapMemorizing part names without understanding what each part does.