Inspecting and Using Air Brakes (Pre-Trip and In-Cab Checks)
Inspecting and Using Air Brakes (Pre-Trip and In-Cab Checks)
This domain tests the air brake portion of the pre-trip inspection and the in-cab tests an examiner will watch. You must know the sequence, the limits, and what a failure looks like.
Core rules
- Check slack adjusters, pushrods, brake chambers, hoses, drums, and linings before the in-cab tests. With brakes released, mark or note pushrod travel; with brakes applied, watch for normal travel and listen for leaks.
- Build-up test: with the engine running, pressure should rise from 85 to 100 psi within 45 seconds.
- Leak test: build to governor cut-out, shut the engine off, release the service brake, and wait one minute. Then apply and hold the service brake for one minute. Loss should not exceed 3 psi for a combination vehicle or 2 psi for a single vehicle.
- Low-pressure warning test: reduce pressure and confirm the light/buzzer activates before 60 psi.
- Parking/spring brake test: with at least 90 psi, apply the parking brake and try to move; the vehicle should not move. With the parking brake released and engine off, fan off pressure; spring brakes should apply automatically before pressure reaches about 20–45 psi.
Worked example
A combination vehicle loses 4 psi in one minute with the service brake applied. That exceeds the 3 psi limit, so it fails the leak test even though it lost only 1 psi in the released check. The driver must find and repair the leak before driving; the released-check result does not cancel the applied-check failure.
Traps
Do not leave the engine running during the leak test. Do not skip chocking the wheels. Do not accept a warning that comes on at 40 psi-it must come on before 60 psi.