In a dual air brake system, what is the maximum allowable air loss rate with brakes applied for a combination vehicle?

The maximum allowable air leakage rate for a combination vehicle (tractor-trailer) with brakes applied is 4 psi per minute. For a single vehicle (straight truck) with brakes applied, it is 3 psi per minute. With brakes released (no application), the limits are tighter: 2 psi per minute for a single vehicle, 3 psi per minute for a combination. Leakage rates above these limits indicate a leak somewhere in the system that must be found and repaired before driving. To test: build pressure to governor cut-out, turn off the engine, apply the brakes (and hold), and time one minute while watching the pressure gauge.