Hi, I think I may be able to help. Sounds like you could have a bad pump discharge check valve. It is usually built into the back end of the air end on the discharge to the oil sump. Also you should try to avoid shutting down the il unit while it is loaded. There is more internal sump pressure when the compressor is under load. Wait for the unit to come up to pressure and unload. You can identify this by listening for the blowdown valve to open and purge excess air pressure from the oil sump. Also check to see if your inlet valve may be sticking open.