After the initial drop, the maximum leakage rate per minute for combination vehicles is:

• A. 3 psi
• B. 5 psi
• C. 4 psi
• D. 6 psi

Answer: (C)

During the air brake leakage check for combination vehicles, you measure how quickly air leaks after the initial drop in pressure once the brakes have been tested. The limit you’re checking against is four pounds per square inch per minute. This reflects the larger, two‑system setup of a tractor–trailer and still ensures there’s enough reserve pressure to stop safely if a leak develops.

If the pressure falls more than four psi in one minute, it indicates an air leak that should be repaired before the vehicle is operated. The value you’re testing against is four psi per minute, which is higher than the three psi per minute allowance for a single vehicle because of the additional lines and potential leak points in combination vehicles.