Air seems to be escaping from the compressor oil refill plug?

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My problem is the compressor motor continues running not building up enough pressure. I have replaced reed valves although problem still continues. The machine itself would not have done 1 hours work and is out of warranty. Air seems to be escaping from the compressor oil refill plug, says Dave.

Red area denotes oil sump. Note sight glass and oil drain in this pump.

In response Willy R. says, well Dave, that air is escaping out of the oil refill plug is the clue that counts, I believe.

Piston (reciprocating) air compressors pump air into the tank using a piston or pistons moving inside a cylinder to draw air in from outside the compressor, and then to pump that air into the tank. This compresses the air as more and more air is pumped into the compressor tank and raises the air pressure.

The piston(s) have rings on them, or seals, or something that is supposed to keep the air on top of the piston rather than letting the air slide between the piston and the cylinder wall.

The air that is bypassing the piston seal / rings has to go somewhere and that’s first into the oil sump, and then up the oil fill tube and out the cap. Since a little air will bypass the seals on even the newest compressor (note: key word here is “little”) the typical oil fill cap will have a small hole in it, allowing any air getting pushed into the sump a place to vent, so that the oil sump does not become pressurized.

If air is noticeably escaping out of the oil fill tube, that’s an indication that more air than should be is blowing past the piston seal and into the oil sump as the compressor pump runs.

The fix? In our opinion you need to tear down the pump and replace the piston seals. If you have not already disassembled the pump, do so. But first, see if you can find a pump repair kit for your model of compressor as not all compressors have parts readily available.

Got a comment for Dave or the response to him, or a question of your own about why air seems to be escaping from the compressor oil refill plug? Add it below.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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