I have an older (appx 1972) Alemite compressor that I picked up from an auto body paint shop. The shop had been closed for awhile and it's been a couple of years since I bought it. It has been indoors all of this time and is in really nice shape. It looks to be about a 180 Gal horizontal tank with two five HP two cylinder compressors. After spending the weekend setting it up, I started it for the first time this afternoon. It starts and runs very well and came up to 180 lbs pressure pretty quickly which isn't all that surprising considering it has two compressors. Each compressor has a pressure relief valve that appears to be diaphragm activated. They look like small versions of power brake vacuum boosters with a small vent hole at the bottom. They both remain closed all the way up to pressure. The problem is that as soon as the unit reaches shutoff pressure and both compressors to turn off, one of the pressure relief valves opens and bleeds off air pressure until the low pressure point is reached and the compressors start up again. Once they start, the pressure relief valve closes and pressure builds. The cycle repeats and always with the same relief valve opening, the other one is fine. Logic would say replace the relief valve but it's an integrated device and the compressor is 45 years old. I have photos if that would help. Does this sound like a valve malfunction, or something else causing the valve to open. I've turned back high pressure shutoff point to 150 lbs from 180 lbe and there isn't a change in behavior.

Any thoughts and suggestione would be appreciated,

Thanks !!!!!

MTR,
You were spot on with the check valve diagnosis. I disassembled it and it was in the very early stages of gumming up. A little "just in time" cleaning and lubrication took care of the problem. Thanks for your assist.
Valzan