Air Brake System Tests: Applied Leakage, Static, and Tractor Protection Valve Tests
Domain 4 focuses on the three formal air brake tests: static leakage, applied leakage, and tractor protection valve. The exam asks you to run each test, read the gauge, and decide pass or fail.
Test rules
- Static: engine off, parking brake released, brakes released; max 3 psi/min single, 4 psi/min combination.
- Applied: engine off, service brakes applied and held; max 4 psi/min for single or combination.
- Tractor protection: build pressure, shut off engine, release parking brake, disconnect emergency line; valve closes and trailer brakes apply.
- Low-pressure warning and spring brake application are checked during the same sequence.
- A leak-test failure means find and repair the leak; do not compensate by pumping.
Worked example
After the applied test, the gauge drops 6 psi in 60 seconds on a single-unit truck. Reasoning: the applied limit is 4 psi/min, so 6 psi/min fails. The static test may have passed, but an applied leak still matters because air loss under braking can reduce stopping power.
Traps that cost marks
Mixing static and applied limits; leaving the engine running; not holding the brake pedal; assuming a small drop is always acceptable; testing tractor protection with the service line.