I have a 375 sullair with the john deere 4239 engine. it starts strong but at high idle and because of it i was told it was burning the starters, after I replaced the starter I disconnected the main line coming from the air tank going to the main idle valve and placed a pressure gauge there and is not giving me any pressure reading, it feels air coming out through that hose but i can put my finger on it and stop it. At the beginning I was told is the air/oil separator because it was making a lot of air, then less averytime until is making just a little bit. It doesn't smell like burn rubber but could it be the unit coupler that is spinning and grabs sometimes and makes little air?? please advise me how I can test the air end unit or what other test I can do before we go any further removing anything.