How should you test the air leakage rate in a parked truck?

To check air leakage: bring air pressure to governor cut-out (around 125 psi), turn off the engine, chock the wheels, release the parking brake, time one minute, and observe the air pressure drop. The maximum acceptable leakage rate is 2 psi per minute for a straight (single) vehicle and 3 psi per minute for a combination vehicle (tractor-trailer). For applied brakes (foot on the pedal), the limit is 3 psi for a single vehicle and 4 psi for a combination. If leakage exceeds these limits, the leak must be found and repaired. This test is part of the standard pre-trip air brake check. Slow leaks under normal driving can drain the system enough to trigger the low-pressure warning during prolonged operation.