The GM tech books state to testityou should first use the fast fill valve and put in 100 lbs of air then use a vacuum source and it should pump up to about 120 lbs. I test my rebuilds of the car.with no compressed air. I have a guage I put on the output side and vacuum pump on the inlet. If working properly you will hear a sort of tick, tick sound back and forth over and over. A steady no tick means it's not working. Also to test the tank etc for leaks you pressurize the unit and have a vacuum hose attached from the inlet filter connection to the vacuum inlet fitting both tight fit, and submerge the whole unit in a bucket of water, look for any bubbles of course.