Old Sears Air Compressor Leaks

[petertay15]

This old compressor leaks air out of the oiler cup. How does air pressure get into the crankcase? And is it fixable? Thanks for replies.

[WJB]

Higher end compressors have an inline check valve between the compressor outlet and reservoir. Unlikely this compressor has one so it relies on the outlet reed or poppet valve to isolate the compressor from the reservoir. If a reed or poppet valve is bad the reservoir air pressure can pass the piston rings and enter the crankcase

[Stwood_]

May have some carbon built up in the reed valve under the head. Depending on the head gasket, I'd do some cleaning of that valve maybe.

[Roly]

This old compressor leaks air out of the oiler cup. How does air pressure get into the crankcase? And is it fixable?  Thanks for replies.

I would think the most likely cause is air is getting past the piston.   Normally if the check vale or reed valves are bad it will have a hard time starting as the cylinder would be pressurized.  While if it was able to start it would be trying to compress already compressed air resulting in much higher cylinder pressures which could get past the piston, unlikely .  I would think the piston seals, a lot like a piston ring, are the problem.   Roly

[daddo]

The check valve and unloader valve should be checked first.  The unloader valve will hiss loudly when the compressor shuts off then the check valve makes sure air stays in the tank and doesn't travel back to the unloader valve/compressor/piston/crankcase.

 A good source;  http://www.about-air-compressors.com/has...ssues.html

 

If those check out ok;
Then most likely have a leaking compression reed valve. See if you can find the model and serial numbers and get a valve kit along with a head gasket- you might as well replace the piston rings while your at it.
Piston rings, even the oil ring are not sealed and have a few thousands gap, so they will leak. Small scratches in the cylinder walls also leak.

 Only oilless compressors have a piston seal as far as I know.

[Roly]

The check valve and unloader valve should be checked first.  The unloader valve will hiss loudly when the compressor shuts off then the check valve makes sure air stays in the tank and doesn't travel back to the unloader valve/compressor/piston/crankcase.

 A good source;  http://www.about-air-compressors.com/has...ssues.html

 

If those check out ok;
Then most likely have a leaking compression reed valve. See if you can find the model and serial numbers and get a valve kit along with a head gasket- you might as well replace the piston rings while your at it.
Piston rings, even the oil ring  are not sealed and have a few thousands gap, so they will leak. Small scratches in the cylinder walls also leak.

 Only oilless compressors have a piston seal as far as I know.

 Any time I have seen check valve or unloader problems, hard or no start problems were the result.   Other than through the piston rings I do not see how it could cause pressure in the crankcase.  Roly

[petertay15]

THANK YOU Daddo for the link to Bill's compressor site. Very helpful as I did not know about the check valve and and unloaded valve. The other answers are appreciated, too. --Peter

[daddo]

 Any time I have seen check valve or unloader problems, hard or no start problems were the result.   Other than through the piston rings I do not see how it could cause pressure in the crankcase.  Roly

 A small enough leak wouldn't pressurize the crankcase enough to cause start problems. The oil fill is open and not able to contain pressure and most likely has a felt pad in it preventing too much rapid blowby.

 At an old age, I might suspect both the unloader, it's controller and/or check valve may not be operating properly.  Not ruling out these easy ones first would be frustrating if one did a tear down to find it was the problem all along.

Unloaders are not all alike- I'm not sure what type he has and what controls it.

[petertay15]

It was a bad check valve. Available at NAPA. So satisfying to have it fixed. THANKS GUYS

[Stwood_]

It was a bad check valve. Available at NAPA. So satisfying to have it fixed. THANKS GUYS