Craftsman 4 Gal pancake air leaking under motor cover

by Robert Mazzarone
(lewes, delaware)

Pin hole in air line plug

Pin hole in air line plug

Pin hole in air line plug
Small hole in air line plug

I have a Craftsman 4 Gal pancake compressor that keeps venting Ir under the plastic motor cover. I removed the cover and there's a 1/4 inch plug in the end of a brass pipe that has a pinhole in the center of it which is where the air is leaking from. What do I need to do to fix it?

Comments for Craftsman 4 Gal pancake air leaking under motor cover

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Jun 17, 2016
unloader valve
by: Bob

Thanks for your response. When I looked closer at the parts, I figured that is what was going on. Evidently my check valve was stuck at least partially open. Other than pulling the plug with the pin hole to investigate, I did nothing and the valve started working again. Problem solved. Thanks again.

Jun 11, 2016
Compressor check valve leaking
by: Bill

You need to better understand how the tank check valve and the unloader valve (in your case pin hole unloader port) work, Bob.

Please see:

Jun 10, 2016
same issue - compressor leaking
by: Bob

I have the same issue and am not following the comments about check valves. Are you saying the check valve is not working and forcing this pin hole to now leak? Mine appears to be an open hole that would leak under any pressure, not just that resulting from a bad check valve. Been using this unit for a few years, but it just started this leak recently. I am tempted to squirt a little silicone cement in that cap to seal it up.

Oct 07, 2015
Leaking compressor
by: Bill

Much grass?

I'm guessing a local colloquialism for muchas gracias? Eh? Senor? :-)

Kidding aside, I think that the groove in the plastic is there on purpose, and it's the unloader valve (using valve loosely) for this compressor.

Sanding out the groove may mean that the air over the piston will be trapped, and on a compressor with lower power supply, or weak caps, may interfere with it starting.

Oct 07, 2015
Mark B:
by: Doug in

Plastic eh?

Wow. Much grass for the tip!

Oct 06, 2015
A Quick Fix
by: Mark B

I know this may not be helpful now, but I have this same model & had the same problem. Mine was also leaking from the check valve out that pin hole. After some poking around, I was able to fix it. I took the larger nut off the check valve.

The only thing inside of it was a spring & a small plastic/nylon type white disc. I noticed a small grove in the disc where it would seat closed.

I figured it had to be the disc, so I took a piece of sand paper & sanded the disc so it was flat & smooth again (until the round groove was gone).

Put it back together & it works like new.

Oct 13, 2014
by: Doug in

I'm inclined to agree with Bill;

I'd just add that if this 'leak' continues for say, more than a minute, then it seems the 'check' in check valve is not working. You may be able to clean and fix it, or it may need to be replaced.

In a normal situation, that plug would be a tube connected to the pressure switch/unloader valve.

Oct 13, 2014
Found the pics
by: Bill

Robert, thanks, I found the pics you sent and added them to the original post.


Oct 13, 2014
Pin hole
by: Bill

Robert, what I surmise, I hope this will be confirmed by others, follows about the pin hole.

In the low cost, mini-compressors, rather than having an unloader valve interfaced with the pressure switch, which would add cost and complexity to the compressor circuit, the cost of the unloader valve is downloaded to the compressor user.

I believe that small pin hole leaks air from the top of the piston all the time. While the compressor is running air is leaking out the hole. And, when the compressor stops, air leaks still, for a while, until all air over the piston is gone, thus providing the function of the unloader valve without having an unloader actually installed.

At least, that's what I think.

Of course, you waste air while the compressor is running, but that waste of air and energy is mitigated by the fact that it's a small hole, and that after the compressor stops and the air over the piston is gone, no more air should leak out.


Oct 13, 2014
Pin hole leaks
by: Robert M.

Bill the model number is 921.152150. I looked over the schematic. The part in question is a check valve. It has a 1/4 inch pipe plug in it that has a pinhole in the center that constantly bleeds air off. Is that hole supposed to be there?

Oct 11, 2014
Craftsman 921152150 ?
by: Doug in

If the title is your model, I can't see what you might be talking about.

I suggest you either start over and post pix, along with correct model number.

Alternatively, you could put pix up somewhere on the web and link to them.

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