Inspecting Air Brake Systems
What the exam tests
Inspection questions check the order of the air brake tests, the numeric limits, and what a failed test tells you.
Standard inspection sequence
- Park on level ground, chock the wheels, and release the parking brake after building pressure.
- Build pressure to governor cut-out (about 125 psi) and shut the engine off.
- Leak test, brakes released: watch the gauge for one minute. Loss should not exceed 3 psi for a single vehicle or 4 psi for a combination.
- Leak test, brakes applied: hold the pedal and watch for one minute; the same 3/4 psi limit applies.
- Low-pressure warning: pump the pedal down; the buzzer or light must activate before pressure falls below 60 psi.
- Spring brake test: continue pumping; the spring brakes should apply at 20–45 psi.
- Compressor build: run the engine; pressure should rise from 85 to 100 psi within 45 seconds.
- Slack adjusters and foundation brakes: with brakes released, check free play; inspect drums, linings, hoses, and drain tanks.
Worked example
After charging to 125 psi and shutting off the engine, the gauge drops from 125 to 115 psi in one minute with the brakes released. Reasoning: a 10 psi loss exceeds the 3 psi (single) or 4 psi (combination) limit, so the vehicle fails the leak test. Find and repair the leak before driving; do not top off the pressure and call it good.
Traps that cost marks
- Running the engine during the leak test; it masks leakage by rebuilding pressure.
- Forgetting to release the parking brake before the released-brake leak test.
- Pumping the pedal too fast to observe the 60 psi warning.
- Checking slack adjuster free play with the brakes applied.
- Confusing the 45-second compressor test with the one-minute leak test.