Inspecting the Air Brake System
This domain tests the three inspection checks emphasized in CDL air brake testing: leakage rate, pressure build-up time, and warning/spring brake operation. You must know the sequence and the numeric limits.
Leakage test
With a fully charged system, engine off, wheels chocked, and parking brake released, apply and hold the service brake for one minute. Loss must not exceed 3 psi for a single vehicle or 2 psi for a combination vehicle. Release the parking brake first; otherwise you are not testing the service circuit correctly.
Build-up test
With the engine at fast idle, pressure should build from 85 psi to 100 psi within 45 seconds. A slower build suggests a weak compressor, a slipping belt, a governor problem, or a leak.
Warning and spring brake check
Pump the brake pedal to reduce pressure. The low-pressure buzzer and light must activate before pressure drops below 60 psi. Continue pumping; the parking brake knob should pop out and spring brakes should apply in the 20–45 psi range. Also drain air tanks, check hoses and chambers, and check slack adjuster/pushrod travel.
Worked example
A combination vehicle holds 100 psi with the engine off and the service brake applied. The gauge drops from 100 to 94 psi in one minute. That is a 6 psi loss, exceeding the 2 psi combination limit, so the vehicle fails the leak test and needs repair before service.
Traps
- Running the engine during the leak test.
- Leaving the parking brake applied during the service-brake leak test.
- Confusing build-up time with leakage rate.
- Checking the warning only at key-on instead of by lowering pressure.
Key points
- Leak limit: 3 psi per minute single; 2 psi per minute combination.
- Build-up: 85 to 100 psi within 45 seconds at fast idle.
- Low-pressure warning must activate before 60 psi.
- Spring brakes should apply at 20–45 psi.
- Drain tanks and check pushrod travel as part of inspection.