[Hugh R]

I believe it may be the pressure relief valve to decompress the cylinder when it stops, so that when the compressor starts up, the cylinder is not under pressure. What it should do is not leak while running and you should have a pressure relief hiss for a second or two when it stops to depressurize the cylinder. Sounds like its defective as it should be closed when running and only open for a second or two on shut off. Try cleaning it or hitting it lightly with a small hammer to see if it seats.