I have a Craftsmen 5.5 25 Gal Air compressor. It come on fine, when the pressure bulds up to normal pressure it cuts off. Then it lets air out to reduce the pressure. Then it cuts back on again. It will not hold the pressure once it pumped it up. I don't know if the regulator or the pressure switch is bad. Any way to tell?
When the compressor shuts off the pressure trips an unloader valve to let pressure off the compressor head. There is a check valve to hold the pressure in the tank while releasing it off the head. My bet would be the check valve is not holding.