Compressor stays at max pressure

How do you mean? it either stops the pump at rated psi or it'll keep pumping and cycle the blow off valve over and over until the motor overheats and shuts off via thermal protection if you're lucky. If you aren't using the psi that's already in the tank and the compressor has been sitting there should be a pull ring on top of the let off valve, you pull that ring to safely empty the tank and also you shouldn't leave the tank loaded with air because there's probably water in there at the bottom of the tank and it can eat through and cause a failure you need to keep it empty when not in use and there should also be a plug for the water. The "regulator" is just a pre-loaded spring diaphragm and contactor switch that shut off the motor you can adjust the spring on it there should be a nut.