Inspecting the Air Brake System: Leak, Low-Pressure Warning, and Tractor Protection Valve Checks
Domain 3 tests pre-trip inspection of air brakes: leak checks, low-pressure warning, and tractor protection valve checks. The exam expects the correct sequence, pass/fail numbers, and what a failure means.
Inspection sequence
- Chock wheels; build pressure to governor cut-out.
- Shut off engine; release the parking brake.
- Static leak check: with brakes released, loss should not exceed 3 psi/min single or 4 psi/min combination.
- Applied leak check: hold service brakes; loss should not exceed 4 psi/min.
- Low-pressure warning: pump brakes; warning must activate before 60 psi.
- Spring brakes: continue pumping; they should apply by 45 psi, often in the 20–45 psi range.
- Tractor protection: disconnect the emergency/supply line; the valve should close and trailer brakes should apply.
Worked example
A combination vehicle loses 5 psi in one minute with the engine off and brakes released. Reasoning: the limit is 4 psi/min for a combination, so this is a failure-do not sign off the inspection. Find the leak before driving.
Traps that cost marks
Forgetting to release the parking brake for the static test; using single-vehicle limits on a combination; pumping the brake with the engine on; not checking warning activation; disconnecting the service line instead of the emergency/supply line for the tractor protection check.