Inspecting the Air Brake System
What this domain covers
Expect questions on the pre-trip air brake inspection sequence: what to look at, the order of the checks, and the pass/fail numbers.
Before you start
Park on level ground and chock the wheels. Inspect slack adjusters, chambers, hoses, and drums for cracks, leaks, or missing parts.
The check sequence
- Build pressure until the governor cuts out, shut the engine off, and release the parking brake.
- Watch the gauge one minute with brakes released: no more than 2 psi loss (single) or 3 psi (combination).
- Apply the service brake fully and hold one minute: no more than 3 psi (single) or 4 psi (combination).
- Fan off pressure with the pedal and confirm the warning light and buzzer come on before 60 psi.
- Keep fanning until the spring brakes apply at 20-45 psi.
- Restart and confirm pressure builds from 85 to 100 psi within 45 seconds.
- Check the ABS lamp, then the pushrod stroke: mark the pushrod with brakes released, apply fully, and measure. More than about 1 inch on a Type 30 chamber means out of adjustment.
Worked example
Left rear pushrod travel measures 1.5 inches. That exceeds the ~1-inch limit, so the vehicle is out of service for brake adjustment until a qualified mechanic corrects the slack adjuster.
Traps that cost marks
- Running the leak test with the engine on.
- Not chocking the wheels or not releasing the parking brake first.
- Measuring the pushrod only in the released position.
- Skipping the trailer chambers and hoses.