Test Air Leakage Rate. With a fully-charged air system (typically 125 psi, turnoff the engine, release the parking brake (push in and time the air pressure drop. The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles. With the air pressure built up to governor cutoff (120 – 140 psi, shutoff the engine, chock your wheels (if necessary, release the parking brake all vehicles, and the tractor protection valve combination vehicle and fully apply the foot brake. Hold the foot brake for one minute. Check the air gauge to see if the air pressure drops more than three pounds in one minute (single vehicle) or four pounds in one minute (combination vehicle. If the air pressure falls more than three psi in one minute for single vehicles (more than four psi for combination vehicles, the air loss rate is too much. Check for air leaks and fix before driving the vehicle. Otherwise, you could lose your brakes while driving.