Unloader valve leaks when compressor running?

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by joe
(ogilvie, mn)

Unloader valve only leaks while compressor is running?

The Unloader valves on two pressure switches
The Unloader valves on two pressure switches


Joe, the air compressor unloader valve is supposed to open when the air compressor reaches the cut out pressure level and the motor stops.

When the compressor starts again, the unloader valve is supposed to close, to ensure that the compressed air that is being generated by the air compressor pump is directed into the tank.

You don’t indicate what your compressor is, or what the pressure switch / unloader valve combination is on your air compressor making it harder to diagnose the problem and solution.

In the photo above you can see the pin that protrudes from the side of the pressure switch. It is this pin that toggles this unloader valve.

So, first you have to make sure that the unloader valve is actually opening and closing by monitoring it while the air compressor runs up to cut out pressure and stops.

Then, if the unloader is actually working, the problem is probably a leaking unloader valve.

Dismantle it, clean it, reassemble it, and test, or just replace it.

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I have a Kellogg 335 that’s leaking air from the unloader only while the air compressor is running. I replaced the schrader valve and it still leaks, just not as much.

OK, that tells you the check valve at the tank and/or the pump valves are working.
When you say Schrader valve, you mean the thing that looks kinda like a threaded ell to compression tube?
Did you adjust it?
If so, then you may have to go for whole centrifugal unloader assembly, or if you’re lucky, you may be able to just clean up the one you have.

Good luck.

I don’t know what adjustments to make so I kept the lock nut in the same position on the “threaded ell to compression tube.”

Thanks for the idea, I’ll get into the unloader assembly and see what I have there.

If you look at the drawing for it, you’ll see the centrifugal mechanism drives the plunger on the valve. I don’t have specs, but I’d guess it’s supposed to be set to just cut off, and then one more turn. It doesn’t look like you need to connect the tube until you get the adjustment right.

I finally had a chance and took the cover off and my unloader was broken! Already got the new one in and it’s working perfectly!

Thanks for the help!

You’re welcome. Thanks for reporting back.
Was there a spec on how to adjust with the part?