Air Brake System Leak Checks
The three checks
- Static leak: fully charged, engine off, parking brake released, wheels chocked. After stabilization, loss must not exceed 2 psi/min for a single vehicle or 3 psi/min for a combination.
- Applied leak: hold the service brake fully applied for one minute. Loss must not exceed 3 psi/min for a single vehicle or 4 psi/min for a combination.
- Low-pressure/spring-brake check: pump down with the engine off; the warning must occur before 60 psi; spring brakes should apply, or the parking knob should pop out, at 20–45 psi.
For combination vehicles, also check trailer lines and tractor protection. Charge the trailer, shut the engine off, and watch the tractor and trailer gauges; a rapid drop suggests a trailer leak. The tractor protection valve should close and protect the tractor supply if trailer pressure falls.
Worked example
A single-unit truck holds 120 psi. Engine off, parking brake released, pressure stabilizes at 118 psi. After one minute it reads 115 psi. That is a 3 psi loss; the single static limit is 2 psi/min, so it fails. If the driver had left the parking brake applied, the test would not be valid.
Traps
The most common errors are measuring during the initial drop, using 3 psi for a single static test, using 4 psi for a single applied test, not holding the treadle fully down, and pumping the brake during a leak test, which changes the reading. Record gauge readings and time; do not estimate.
Valid test conditions
Engine off, fully charged system, wheels chocked, parking brake released for the static test, and a full steady pedal for the applied test.