A straight truck or bus air brake system should not leak more than how many psi per minute with the engine off and brakes released?

• A. 1 psi per minute
• B. 2 psi per minute
• C. 3 psi per minute
• D. 4 psi per minute

Answer: (B)

The test is about how well the air brake system holds pressure when nothing is actively compressing air. With the engine off and the brakes released, there should be only a tiny loss from leaks, not a noticeable drop in pressure. For a straight truck or bus, the allowable leakage is 2 psi per minute. This threshold keeps the system dependable during stops and while waiting, ensuring you’ll have enough stored air when you need to brake again after starting the engine. If the pressure falls by more than 2 psi in one minute, it signals a leak somewhere in the system—such as hoses, fittings, brake chambers, or valves—that needs inspection and repair. The other numbers would either miss small leaks or indicate unsafe levels of leakage.