With the engine off and the brakes released, at which rate should a straight truck or bus air brake system NOT leak?

With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for 1 minute. The air pressure should not drop more than:
- 2 psi for single vehicles.
- 3 psi for a combination of 2 vehicles.
- 5 psi for a combination of 3 or more vehicles.
[California Commercial Driver Handbook]