What is the maximum air leakage rate allowed for a combination vehicle (engine off, brakes on)?

A 4 psi per minute

explanation

For a Class A combination vehicle, if the power unit is equipped with air brakes and the trailer is equipped with electric/surge brakes, the pressure drop should be no more than 3 psi.
• 3 psi for single vehicles.
• 4 psi for a combination of 2 vehicles.
• 6 psi for a combination of 3 or more vehicles.
[California Commercial Driver Handbook]