Inspecting the Air Brake System
Pre-trip order
- Chock the wheels; release the parking brake only when safe.
- Build pressure to governor cut-out.
- Shut the engine off; let pressure stabilize; check the static leak.
- Apply and hold the service brakes; check the applied leak.
- Pump down with the engine off; verify the low-pressure warning before 60 psi.
- Continue pumping; verify spring brakes apply or the parking knob pops out between 20 and 45 psi.
- Drain reservoirs; check compressor buildup, governor cut-in and cut-out, slack adjusters, chambers, hoses, and drums.
Compressor buildup should reach 85 to 100 psi within 45 seconds on dual systems; larger tanks may take longer. Governor cut-out is about 120–125 psi and cut-in about 100 psi.
Worked example
A combination vehicle is fully charged, engine off, parking brake released. After the initial drop stabilizes, the gauge loses 4 psi in one minute. The static limit is 3 psi/min for a combination, so this fails. If the same vehicle lost 4 psi while the service brakes were fully applied, that would be within the 4 psi/min applied limit-but the test must be run correctly.
Traps
Testing with the engine running masks leaks. Not waiting for stabilization inflates the first-minute drop. Using single-vehicle limits on a combination is wrong. Forgetting to chock the wheels is unsafe. Skipping the low-pressure and spring-brake checks because the gauges look fine loses marks.
Key inspection evidence
Record the gauge reading and time, note governor cut-in and cut-out, and confirm the warning and spring-brake actions actually occurred.