by Johan
(Lund, Sweden)


Leaking unloader valve when compressor is running? I have a problem with a compressor pressure switch which has a leaking unloader valve. The valve lets out air immediately when the compressor is started. Due to this leak the compressor never reaches its turn-off pressure. I have tried replacing the pressure switch but I get the same problem also with this component.

I have checked the one-way valve, which looks fine. (As I have understood it this valve is not likely to be the problem anyway, since the unloader valve lets out air already while the compressor is running?) My suspicion is that I have done something wrong when mounting the pressure switch (particularly when mounting the unloader tube).

I don’t see what this error might be though. The tube is just “inserted” into place – maybe I’ve misunderstood something there?

Would you have any suggestion for how to troubleshoot further?


Compressor pressure switch with unloader valve shown on the right side of the image.

Compressor pressure switch with unloader valve shown on the right side of the image


Johan, if you haven’t had the chance to review the page on this site, here is the info on unloader valves.

That your unloader valve is leaking air when the compressor is running indicates to me, since as you point out they are a simple, spring operated check valve, that you have something holding the ball open, allowing the air to escape.

The unloader valve is often opened (frequently by a lever moved by the pressure switch) by the depressing of a pin actuator that pushes the ball off seat inside the valve and allows air to flow.

In your case it sounds like either the ball is lodged off-seat by debris or something that is preventing the ball to seat and shut off the outflow while the compressor is running, or the linkage with the pressure switch is somehow compromised, resulting in the pin being depressed when the compressor is running rather than when it’s shut off.