During the Air Leakage Rate Test on a combination vehicle (tractor-trailer), the engine is off, the key is in the 'on'/'battery charge' position, the parking brake and tractor protection valve are released, and the foot brake is fully applied and held for one minute. What is the maximum allowable air pressure loss during that minute?

nhcdm-s.pdf Section 11M.1.2 - Air Brake Check, p. 11M-3; 49 CFR 383.111(b)(5)

A leak rate above these thresholds means the compressor may not keep up with air loss during a long descent or repeated braking, risking total air pressure loss and brake failure. Examiners fail this test on sight because sustained leakage, even if slow, compounds over a trip.