CDL In-Cab Air Brake Test, Step by Step
Last updated · By David Nelson
The in-cab air brake check proves your air system holds pressure, warns you when it is low, and applies the brakes on its own before pressure is gone.
For a Class A combination: build air to governor cut out at 120 to 140 psi, hold the brake and lose less than 4 psi in one minute, get the low air warning before 55 psi, and see the parking brake valves pop out between 20 and 45 psi. Then test the parking and service brakes.
Step 1: Build air and check governor cut out
With the engine running, let the pressure build. Say: "The governor cuts out between 120 and 140 psi." Some examiners also want to see the governor cut back in, usually around 100 psi.
Step 2: Applied pressure leak test
Shut the engine off, turn the key back to on, and release the parking brakes by pushing in both valves. Let the needle settle, then press and hold the brake pedal hard for one full minute.
Say: "For a combination vehicle, the pressure should not drop more than 4 psi in one minute." A single truck allows 3 psi.
Step 3: Low air warning
Fan the brake pedal to drain air. Say: "The low air warning light and buzzer come on before 55 psi." Keep fanning.
Step 4: Spring brakes pop out
Keep fanning until the yellow and red valves pop out. Say: "The tractor protection valve and parking brake valves pop out between 20 and 45 psi."
Step 5: Parking brake check
Start the engine, build air, and set the parking brakes. Put it in a low gear and gently pull against the brakes. Say: "The parking brakes hold the vehicle." Then do the trailer brake test the same way with only the trailer brake set, if your state asks for it.
Step 6: Service brake check
Release the parking brakes, roll forward at about 5 mph, and press the brake firmly. Say: "The truck stops straight with no pulling to either side and the pedal feels normal."
Frequently asked questions
What psi should the governor cut out at?
Between 120 and 140 psi on most trucks. Say the range out loud while you point at the gauge.
How much air loss is allowed in the leak test?
With the brakes applied, a combination vehicle can lose no more than 4 psi in one minute. A single vehicle can lose no more than 3 psi.
When should the low air warning come on?
Before the pressure drops below 55 psi. On some older vehicles the manual lists 60 psi, so check your state manual.
At what pressure do the brake knobs pop out?
The parking brake and tractor protection valves should pop out between 20 and 45 psi.
Is the air brake check an automatic fail?
In many states, missing a step or failing to say the numbers on the in-cab air brake check is an automatic fail of the whole inspection.
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