Air Brake System Leaks and Pressure Loss
The Leaks and Pressure Loss domain tests the numbers: how much pressure loss is acceptable, how fast the compressor must recover, and when a leak makes the vehicle unsafe to drive.
Static and applied leak limits
- Fully charged, engine off, brakes released: no more than 3 psi per minute for a single vehicle and 4 psi per minute for a combination.
- Fully charged, brakes applied and held: no more than 4 psi per minute single and 6 psi per minute combination.
- A steady hiss, a gauge falling faster than those rates, or a compressor that cannot keep up all mean the vehicle stays parked.
Compressor performance
- At fast idle, the system should build from 85 to 100 psi in roughly 45 seconds for a belt-driven compressor.
- Slow build-up points to a worn compressor, a slipping belt, or a leak that the compressor is chasing.
Reading the gauge
Judge leakage after pressures stabilize, not while the needle is still moving from the last pedal application. A gauge that keeps falling with the engine off is showing a leak. Failed chamber diaphragms and leaking slack adjusters usually show up in the applied test.
Worked example
A tractor-trailer with a fully charged system and brakes released loses 6 psi in one minute. The limit for a combination is 4 psi per minute, so it fails. The same rig with the brakes applied loses 5 psi in one minute, which is inside the 6 psi applied limit - but the earlier failure still grounds the truck.
Traps
Using single-vehicle numbers on a combination; running the test with the engine idling; timing less than a full minute; treating "the compressor keeps up" as proof there is no leak; forgetting that both tests must pass.