Air Brake System Inspection and Leak Tests
The inspection domain tests whether you can run the three pressure tests in the right order and quote the numbers.
The three pressure tests
- Compressor build-up. With the system charged and the engine at fast idle, pump down to about 85 psi and time the rise to 100 psi. It should take no more than 45 seconds; slower means a weak compressor, slipping belt, or a leak.
- Applied leak test. Engine off, wheels chocked, parking brake released, at least 100 psi in the system. Hold a full service brake application for one minute. Loss must not exceed 3 psi on a single vehicle or 4 psi on a combination.
- Warning and spring-brake test. Engine off, fan the brakes down and watch the gauge. The low-pressure light and buzzer must come on before pressure falls below 60 psi. Keep fanning: the spring brakes must fully apply between 20 and 45 psi.
Visual checks
Measure push rod travel at each chamber with the brakes applied. Typical limits are 2 inches for standard-stroke type 30 chambers and 2½ inches for long-stroke chambers, with smaller limits for smaller chambers - check the chamber stamp. Linings must be at least ¼ inch thick at the shoe and not oil-soaked. Look for cracked drums, chafed hoses, loose chambers, and a slack adjuster that moves more than about an inch by hand.
Worked example
A combination loses 9 psi during the one-minute applied test. The limit is 4 psi, so the unit fails. The driver may not accept it; the leak must be found - commonly at a chamber, a hose fitting, or a trailer gladhand - and repaired before dispatch.
Traps
Do not run the applied leak test with the parking brake set; the spring brakes will mask a service-circuit leak. Do not test the warning device by opening a drain cock with the engine running, because the compressor keeps up.