CDL Air Brake Tests and Warnings

A visible and/or audible warning signal must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles).

Study focusRecognize low air pressure warnings and choose the safest response.

Common trapIgnoring a low air warning or waiting until spring brakes apply.

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.

For a static leak test (brakes released, engine off), a combination vehicle should not lose more than 3 psi in one minute. A single vehicle should not lose more than 2 psi in one minute.

Study focusApply the correct static and applied air leakage test limits.

Common trapMixing up single-vehicle and combination-vehicle leakage limits.

During an applied leak test (engine off, service brakes fully applied by pressing the brake pedal), a combination vehicle must not lose more than 4 psi in one minute.

Study focusApply the correct static and applied air leakage test limits.

Common trapMixing up single-vehicle and combination-vehicle leakage limits.

To test the warning signal, turn the engine off, leave the key in the 'on' position, and rapidly step on and off the brake pedal. The warning signal must activate before pressure drops below 60 psi.

Study focusRecognize low air pressure warnings and choose the safest response.

Common trapIgnoring a low air warning or waiting until spring brakes apply.

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.

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.

To check slack adjusters, you must release the parking brakes so there is no tension on the system. If a slack adjuster moves more than about one inch when pulled by hand, it probably needs adjustment.

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

Common trapAssuming parking brakes work independently of air pressure behavior.

For a single vehicle (like a straight truck or bus), the static air leakage rate must be less than 2 psi in one minute.

Study focusApply the correct static and applied air leakage test limits.

Common trapMixing up single-vehicle and combination-vehicle leakage limits.

Some vehicles have a control handle on the dashboard that allows you to apply the spring brakes gradually. This modulating valve gives you more control when using spring brakes for an emergency stop.

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

Common trapAssuming parking brakes work independently of air pressure behavior.

Spring brakes rely on the mechanical force of powerful springs. Air pressure is actually used to hold the springs back; when air pressure is released, the springs expand and apply the brakes.

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

Common trapAssuming parking brakes work independently of air pressure behavior.

Some vehicles have a separate air tank with its own control valve to release the spring brakes temporarily. This provides enough air to move the vehicle to a safe location if the primary air is lost.

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

Common trapAssuming parking brakes work independently of air pressure behavior.

To test the automatic application of the spring brakes, fan off the air pressure by pumping the brake pedal. The parking brake valve should pop out automatically between 20 and 45 psi.

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

Common trapAssuming parking brakes work independently of air pressure behavior.