Air Brake System Checks (Leakage, Low Pressure Warning, Spring Brakes)
This domain tests the specific pass/fail air brake checks: leakage, low-pressure warning, spring brake operation, and compressor buildup. The exam wants numerical limits and correct test conditions.
Leakage and pressure checks
Fully charge the system, shut the engine off, chock the wheels, release the parking brake, and hold the service brake. Air loss should not exceed 3 psi in one minute for a single vehicle or 4 psi for a combination. With the engine running, pressure should build from 85 to 100 psi within 45 seconds. The low-pressure warning must activate before the gauge falls below 60 psi. Spring brakes should apply automatically between 20 and 45 psi. The safety valve is set near 150 psi and is a backup, not a normal pressure regulator.
Worked example
A combination vehicle’s gauge drops 5 psi in one minute with the service brake held. That fails: the limit is 4 psi. The driver should not sign off the inspection; find the leak, repair it, and retest. If the same vehicle dropped 3 psi, it would pass the applied leakage limit for a combination.
Traps that cost marks
Do not test leakage with the parking brake applied or before full charge. Do not treat 5 psi as acceptable on a combination. Do not expect the warning at 20 psi; it must occur before 60 psi. Do not confuse the spring brake application range (20–45 psi) with the warning range. Do not use the safety valve to set normal system pressure.