During an applied air leakage test for a single vehicle, after fully applying the brakes, how much pressure drop per minute is generally acceptable?

In an APPLIED air leakage test (engine off, system charged, then fully APPLY and hold the brakes), the pressure drop should not exceed about 3 PSI per minute for a SINGLE vehicle (or about 4 psi per minute for a COMBINATION vehicle). CDL air brakes inspection.

Compare: in the STATIC (brakes released) leakage test, the limits are lower — about 2 psi (single) / 3 psi (combination) per minute. EXCESSIVE leakage indicates a problem (leak in lines, chambers, valves) that must be fixed before driving. The brakes-applied test reveals leaks that only appear under application. Knowing the acceptable applied-leakage rates (3 psi single / 4 psi combination) is commonly tested air brake inspection knowledge.