Inspecting the Air Brake System
This domain tests the pre-trip inspection sequence for air brakes: building pressure, checking compressor cut-in and cut-out, leakage, warning devices, spring brakes, and the mechanical condition of chambers, slack adjusters, and lines.
Inspection order
Start with a fully charged system and wheels chocked. Build pressure and note governor cut-out, usually about 120–125 psi, and cut-in near 100 psi. The compressor should raise pressure from 85 to 100 psi within 45 seconds. With the engine off and the parking brake released, hold the service brake and watch the gauge; leakage should not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination. The low-pressure warning must work before 60 psi, and spring brakes should apply at about 20–45 psi. Drain the reservoirs daily and inspect hoses, glad hands, chamber clamps, push rods, and slack adjusters.
Worked example
During pre-trip, pressure climbs from 85 to 100 psi in 60 seconds. That fails the 45-second buildup check. Possible causes include a weak compressor, a slipping belt, a governor stuck open, or an air leak. Do not pass the vehicle until the cause is found.
Traps that cost marks
Do not run the leakage test with the parking brake applied. Do not test buildup with the engine off. Do not accept 5 psi per minute on a combination. Do not assume the warning must wait until exactly 60 psi; it must come on before that point. Do not skip slack adjuster and push rod checks.