The air brake portion of the CDL skills test pre-trip inspection is one of the most-failed components of the entire CDL examination. The good news is that it is also one of the most studyable: the script is finite, every item is checkable in advance, and good preparation produces a near-certain pass.
The static leakage test
With the engine off, full air pressure built up, and the trailer (if applicable) connected, fully apply the brake pedal and hold it for one minute. The acceptable air loss is two pounds per square inch per minute for a single vehicle, three pounds per square inch per minute for a combination. Verbalize the test as you perform it.
The low-pressure warning test
With the engine off, repeatedly fan the brake pedal to drop air pressure. The low-pressure warning device must activate at sixty pounds per square inch — never lower. Most warning devices are a buzzer and a dashboard light; some are a "wig-wag" mechanical flag.
The spring brake activation test
Continue fanning the brake pedal to drop pressure further. The spring brakes must apply between twenty and forty-five pounds per square inch. You will hear and feel the spring brakes set; the parking brake control will pop out automatically.
Air compressor governor cut-in and cut-out
With the engine running, the air compressor should cut out (stop building pressure) at approximately one hundred twenty to one hundred forty pounds per square inch and cut back in at approximately one hundred to one hundred ten pounds per square inch. These numbers are vehicle-specific; verify against the manufacturer's specifications for your test vehicle.
Slack adjusters and pushrod travel
For each wheel position, demonstrate that the slack adjuster is properly adjusted: pushrod travel under firm pressure should not exceed approximately one inch on Type 30 chambers (the most common type). Excessive travel indicates an out-of-adjustment brake that will not generate full braking force.
For drivers preparing for Air Brake Pre-Trip Inspection Checklist (Printable), additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.