Air Brake System Inspection and Maintenance Checks
The Inspection and Maintenance Checks domain tests the in-cab air brake test and the visual checks a driver must make before a trip. Exam questions often turn on the order of the test and the exact pressure limits.
The in-cab sequence
- With the engine running, build pressure to governor cut-out (about 125 psi), then shut the engine off.
- Release the parking brake and watch the gauge for one minute: leakage should not exceed 2 psi in a single vehicle or 3 psi in a combination.
- Apply and hold the service brake for one minute: leakage should not exceed 3 psi (single) or 4 psi (combination).
- Turn the key on and pump the brakes down; the low-pressure warning must sound or light before pressure falls below 60 psi.
- Keep pumping; the spring brake (parking) valve should pop out between 20 and 45 psi.
- Restart, rebuild pressure, and test that the parking brake holds and the service brake stops the vehicle.
Visual and maintenance checks
Check slack adjuster pushrod travel (about 1 inch or less on most S-cam brakes), hoses and lines for cracks or chafing, the compressor belt, and the air dryer. Drain all reservoirs daily to remove moisture and oil. A buildup of water or oil points to a failed dryer, compressor, or evaporator.
Worked example
A tractor-trailer charges to 120 psi. With the engine off and the parking brake released, pressure falls to 116 psi in one minute, a 4 psi drop. The limit for a combination is 3 psi, so the vehicle fails the static leak test and must not be dispatched until the leak is found.
Traps that cost marks
- Running the leak test with the engine on or the parking brake set.
- Using the single-vehicle limits on a tractor-trailer, or the reverse.
- Testing the low-pressure warning without the key on.
- Assuming a slack adjuster needs no attention because it is automatic.