unloader valves

I have a curtis 20 hp I rebuit valve assembly had one plate disentergrated unloader valve were stuck so I freed them up and the pin was lose from the part that set on the plate so I drill and pined them in place started compressor and it slowly pumped up to 120 would go no future I need to know the exact funtion of the unloader valves could I remove the plunger and it work or the plates suppose to be held down when running The unloader had never had any piping on them so I piped them to gether and ran them to back flow valve and still nothing works compress would not even pump to over 20 lb before this or after second starting If you could give me a nuber I could call you at and explain more I'd apreciate iy I don't know any one in this area that works on compressor Thanks Marion