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Check to see if this compressor uses what's called an "oil stop" valve. This is simply a check valve that prevents the backflow of pressurized air and oil from the separator tank, back into the screwset at low rpm and shutdown. It may be a simple swing check valve, or a spring loaded steel plate and seat type valve. Does your pressure drop quickly if you idle the compressor down without a load? Alternately, your intake valve may not be closing properly. Check for a pinched air signal line, or a line that has been disconnected in any way. Take off the inlet filter and observe the action of the inlet valve when the unit is idled down. Screw compressors use either a normally open, or normally closed inlet valve. If normally closed, the valve itself sometimes performs the operation of the oil check valve, so if it does not fully close, or the seat is worn, you will get oil flow back into the intake. Check that any cup seals in the intake valve shaft has not failed. Is this a poppet type inlet valve, or a ball type inlet valve? I have seen simple ball valves used as inlets on a Sullair, it might have worn out seals in it. Is the compressor overfilled? Does your separator tank blow down valve work when you shut off the compressor?
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