During a static air leakage test (engine off, brakes released), how much air loss per minute is generally acceptable for a single vehicle?

In a STATIC AIR LEAKAGE test (engine off, system charged, brakes released), air loss should not exceed about 2 PSI per minute for a SINGLE vehicle (or about 3 psi per minute for a combination vehicle). CDL air brakes pre-trip. With the brakes APPLIED, the allowable rates are slightly higher (about 3 psi single / 4 psi combination per minute). EXCESSIVE leakage indicates a leak in the system that must be repaired before driving — too much leakage could cause loss of braking. Knowing the acceptable air leakage rates (and that excessive leakage means a problem) is commonly tested air brake inspection knowledge.