The air leakage rate for a combination vehicle (engine off, brakes off) should be no more than ____ PSI per minute.

With a fully-charged air system (typically 125 psi), turn off the engine, release the parking brake (push in); and time the air pressure drop. The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles.
[Commercial Driver's License Manual]