Pre-Trip Inspection of the Air Brake System
This domain covers the sequence a driver must actually perform before driving: draining tanks, testing for leaks, checking build rate, checking the governor, checking the warning devices, and checking the spring brakes. Examiners expect the correct order and the correct numbers.
The test sequence
- Drain the tanks first, with the engine off and pressure up, and watch for moisture or oil.
- Static leak test, brakes released: with the engine off and wheels chocked, note the gauge, wait one minute, and recheck. Allowed loss is about 3 psi (single vehicle) or 4 psi (combination).
- Applied leak test: hold the foot valve fully down for one minute. Allowed loss is about 4 psi (single) or 6 psi (combination).
- Build rate: at operating rpm, pressure should climb from 85 to 100 psi within 45 seconds.
- Governor check: confirm cut-out and cut-in by watching the gauge while the compressor cycles.
- Warning devices: fan the brakes down and confirm the light and buzzer come on before 60 psi.
- Spring brake test: keep fanning; the spring brakes must apply in the 20–45 psi range (and the tractor protection valve must close on a combination).
- Physical checks: slack adjuster travel, push rod stroke, hoses and glad hand seals, ABS light, brake lights.
Worked example
On a tractor-trailer, the released test loses 5 psi in one minute and the applied test loses 5 psi in one minute. The released test already fails: the combination limit is 4 psi. A driver who only ran the applied test would have passed it and missed the defect.
Traps that cost marks
Testing with the engine running or the wheels unchocked; using a single-vehicle limit on a combination; checking the warning light while pressure is still high; skipping the slack adjuster and push rod inspection because the gauge looked fine.