Unloader not functioning (or not present) ?

by Lindsy

Compressor pump zoomed out

Compressor pump zoomed out

Compressor pump zoomed out
Compressor pump zoomed in
Compressor motor detail
Blue air compressor modified - updated image

I acquired this compressor a while ago. It starts and runs and fills up the tank normally. When it shuts off, there is no audible evidence of an unloader functioning. Indeed, the line retains pressure and when air is used and the pressure drops and the compressor tries to run again, it is unable to - the belt slips or the motor hums.

I do not see an unloader in the switch. Is it possible that it was originally built with a motor strong enough to overcome the residual pressure ? Is it possible that there is an unloader in the switch that I am simply not recognizing and is malfunctioning ? Is there some kind of unloader function in the part that attaches to the tank (with the pressure gauge, line from the compressor, and line to the pump) ?

Any help would be appreciated.

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Jul 12, 2018
Power cord / pic
by: Lindsy

Doug - IIRC the cord is a generic electric dryer / stove cord from Home Depot.

(Moderator - I believe Doug was referring to the "after" pic at...

Noted and fixed - the new image is the last on the right - Moderator)

Might be useful as a comparison to the "before" pic at the top of the thread (for newbies like I was)

Jul 11, 2018
Nice job!
by: Doug in s.d.ca

Thanks for sharing.

Maybe Bill can grab that pic and put it up here?

What's the power cord? For a stove or something?

(New photo has been uploaded, last photo on the right - Moderator)

Jul 11, 2018
Current status
by: Lindsy

Upon removal of the crankcase cover I discovered that the counterweights that are supposed to activate the unloader were broken off and laying in the sump.

With no factory replacement parts available I purchased and installed a cheap switch with an incorporated unloader.

I also lapped the check valve with valve grinding compound and it now works well enough to retain pressures for several hours - it still bleeds off overnight.

Many thanks to this forum for general knowledge and the help from Doug in getting this compressor in service.

Mar 09, 2018
Maybe you could lap it,
by: Doug in s.d.ca.

enough to get a seal. You might find one on fleabay or whatever. But not real likely.

I think probably the solution is to add a second check valve somewhere between that one and the tank...

Mar 08, 2018
by: Lindsy

As I don't see any way to add additional pictures to my post, here are links:
Disassembled unloader -
Check valve -

Mar 08, 2018
And now the check valve !
by: Lindsy

Doug - thank you. Misinformation elsewhere on the web led me to believe that the component on the end of the head unit crankshaft was not the unloader. It is indeed and upon removal and disassembly I discovered the pin that is supposed to move in and out to relieve pressure was frozen solid - it took banging on it with a mallet to dislodge it from the housing ! After a thorough cleaning I reassembled the unloader. While I was at it, I pulled the check valve as it had a very small leak bleeding pressure very gradually. Unfortunately, upon reassembly and testing it now has a somewhat significant leak - apparently the built up crud on the check valve and seat makes a better seal than a cleaned up check valve and seat !!
The plate on the tank states that this is a compressor from CURTIS MFG. CO., ST. LOUIS, MO., built in 1969.
Is there any chance that I will be able to purchase a new seal (the white part on the valve) for the check valve ? The local Curtis distributor did not have anything resembling it in stock.

Feb 18, 2018
by: Doug in s.d.ca.

It's there. And either broken or just stuck.

The block near the pressure gauge is almost certainly a check valve, which closes the tank off from the pump. The quarter inch tube from there back to the pump is where air in the large delivery tube goes back the the unloader in the pump. So you could start by removing that tube, and maybe put a manual valve on it. If you then let it pump up, it will behave as it is now (with the valve closed. Then open the valve, and you'll get some air. Close it again, and the pump will be able to restart.

If that works, you'll want to get into the pump to see what's going on in there - it'll be some centrifugal arrangement that's suppose to be open when the pump stops, and close again when the pump restarts.

Good luck and let us know how you fare, please.

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