Compressor leaks down..... grrrr

cathead

CATWERKS LTD
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My compressor leaks down over several days. If I'm not actively using it I shut off the breaker but it is a big energy waster.
I have checked all the fittings with soapy water and can't find any leaks. The pressure is escaping somewhere. Is there
a valve where the pressureized air enters the tank that leaks back into the compressor head maybe? It's a Sanborn 5hp 220V 80 gallon
compressor, about 15 years old. This condition seems to gradually occured over the last year or so.
 
I think it could be the unloader valve. It releases pressure from the head so the motor does not start under load. look for the tube going from the head to the tank.
 
You have a compressor check valve that goes into the tank, the valve actually sits in the tank and then connects to the HP pump side outlet. The valves get crud/rust in them or just stick and do not seal. Most of these come with a small side port which goes to an unloader which bleeds air off the HP pump side until it comes to speed. I just replaced this valve in my compressor which was less than a year old. They used some iron pipe on the HP side which rusted and fowled the valve. Replaced it with brass.

https://www.jackssmallengines.com/equipment/list/air-compressor/Check-Valves/254:Sanborn
 
You have a compressor check valve that goes into the tank, the valve actually sits in the tank and then connects to the HP pump side outlet. The valves get crud/rust in them or just stick and do not seal. Most of these come with a small side port which goes to an unloader which bleeds air off the HP pump side until it comes to speed. I just replaced this valve in my compressor which was less than a year old. They used some iron pipe on the HP side which rusted and fowled the valve. Replaced it with brass.

https://www.jackssmallengines.com/equipment/list/air-compressor/Check-Valves/254:Sanborn
That sounds about right. I will be ripping into it soon. Thanks!
 
I have had problems with the in-line check valve. The unloader valve will not provide enough flow to create enough pressure drop to properly seat the check valve. In my case, there is a very distinct hiss of escaping air but I could see a minor leak being unnoticeable. You should see a leak with a soap bubble test. If it continues for more than a few seconds, the check valve is faulty.
 
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another place to check is the unloader valve diaphragm, Some of these have several small screws that over time can let air slowly leak from under the diaphragm. Good hunting.

Jack
 
My compressor also has a drain valve in the bottom of the tank. If I'm not careful in closing it (it's hard to reach), it can leak slowly as well.
 
If it take 2 to 3 days to leak down I would not waste my time or money to mess with it at this time. When you can see the PSI drop between cycles it's different, you have a problem. Just my opinion.

fixit
 
My compressor is a 30 gallon Sanborn. Other possible leak sources would be the pressure switch diaphragm or the manual pressure release. A pinhole in the diaphragm or a bad gasket would be hard to spot. Other possibilities are the main pressure gauge or the pressure regulator. Again, internal leaks would be hard to spot.

An 80 gallon tank holds about 10 cu. ft. or some 100 cu.ft. of uncompressed air. A leak down time of 50 hrs. would be around 2 cu. ft./hr. which is a fairly substantial leak. I would use a mechanic's stethoscope to try to locate the leak. Not having a stethoscope, a length of flexible tubing will work.
 
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