What should the air loss rate for a straight truck or bus (engine off, brakes on) NOT exceed?

A 3 psi in one minute.

explanation

The driver then holds the foot brake for 1 minute after stabilization of the air gauge. The driver checks the air gauge to see that the air pressure drops no more than 3 pounds in one minute (single vehicle) or 4 pounds in 1 minute (combination vehicle) and listens for air leaks. [California Commercial Driver Handbook]