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Old 01-06-2008, 01:13 PM
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Compressor Pump Leak....

Well, I scored a nice 60gal compressor with a 5hp motor, in awesome shape, traded my buddy for a few yards of material . The Pump was dead and gone, so I picked up a new 2cyl. one got her all up and running. Once the pump shut off, I noticed that the air is leak back through the pump, out the vent on the pump. Is there a break in time on the pumps before they seal, or anything like that. Or should I be returning this pump? Ideas in general?
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Old 01-06-2008, 01:24 PM
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Sure seems like there should be a one way valve between the pump and the tank to prevent leak back.
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Old 01-06-2008, 09:33 PM
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Yep there is supposed to be a valve that keeps the pressure from leaking back into the pump and this thing is for more reasons than just the air leaking past the pump. In order for this pump to be properly installed you need that valve, usually located where the pipe from the pump enters the tank, and a pressure relief valve as part of the pressure switch. The tank valve will prevent air from flowing back into the pump while at the same time the relief valve (unloader) will bleed the pressure from the pump head and the line from the pump to the tank. All of this is VERY important because the air pressure loss through the pump is the minor problem, the real problem here is that there will be pressure on the pump when the motor tries to restart it causing an overload and possible (probable) damage to the motor due to the excessive torque needed.
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Old 01-06-2008, 09:52 PM
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Valve eh? Gotcha..... like I said, the pump was missing on the pump when I got it, so didn't get it, nor have I put together a compressor before. I'll have to pick one up.
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Old 01-07-2008, 07:30 AM
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Don't be surprised if that valve is hard to find because they are usually model specific and have to be ordered from a parts list. What kind of compressor was this thing when new? What brand of pump is the replacement? When you got the compressor was the line from the pump to the tank still attached? The valve is usually lost when this line is replaced and some people inadvertently throw it away thinking it is just a fitting on the line. Do you still have the small line (usually plastic) that runs from the pressure switch to the pump head or pump/tank line? If this line is missing there should be a small brass fitting on the switch where it is suppose3d to be attached. If all this stuff is gone, which is common on compressors with a missing pump, it may be easier and cheaper to just plumb in a one-way check valve at the tank and then put a tee somewhere in the line between the pump and the tank then route a small line to the pressure switch (assuming you have the right switch). DON'T leave off this small line! If the switch does not have a provision for the unloader then most likely it is the wrong switch and this is probably the leading cause of motor failure. When a switch goes bad a lot of times someone will simply go to a hardware or plumbing supply and buy a pressure switch not designed for a compressor and this line will get left off. But if you do this then when the compressor tries to start it will have to start the pump while it is under a load from the pressure in the tank which will result in a motor overload. Sometimes the motor will stall and fail to start the pump so it will be obvious there is a problem but a lot of the time it will seem to start ok and the first sign something is wrong is when the motor burns out!

Last edited by oldred; 01-07-2008 at 07:43 AM.
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Old 01-07-2008, 11:46 AM
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Thanks for all the good info Oldred, the old unit was Devilbiss, wish he wouldn't have chucked it, it probably could have been fixed. New one is just a princess auto special, so made in china, but for how often I use it and what the budget allows, it will do for now. Small line is there, its hooked up from the tank to the pressure switch, hope thats right. Line is there, just no valve, it must have been left on the old pump. So I'm thinking all I need is the check valve, will have to go look around today.
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Old 01-10-2008, 09:21 AM
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My compressor started doing that once and I removed the valve and found a piece of crud in it that was causing it to leak back slowly.

Cleaned it and never had a problem since. And, that was about 14 years ago!
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Old 01-10-2008, 09:44 AM
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Actually, with a little more investigation, I.E me removing the tank feed line, I found the valve, it screws into the tank and the line connects to it. The old one was fubared, so Acklands Grainger ordered me a new one, will be here this morning.
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