This house came with a workshop. In that workshop is a Binks air compressor. Model 331010, 30 gal tank, 3/4 hp motor, wired for 220 single phase.

It runs very well. However, it has a slow leak. I sealed off all the leaks I could find in the piping system (which runs around the shop/garage areas), but still the thing would lose about 10-20 lbs pressure a day with the motor off. Finally I closed the main valve between the compressor and the distribution plumbing (should have done that first, yes), and determined the leak in in the compressor itself.

The main suspect appears to be the "unloader valve", the purpose of which is to relieve pressure in the cylinder head after off pressure has been reached. This is so the compressor can start up again without fighting against retained pressure.

Naturally there's no manual for the thing and I can't find any online. I'm guessing the compressor is from the 70's. It says Binks all over it, but I gather that Quincy made it.

Most discussion of unloader valves shows them attached to the pressure switch. But this one doesn't have the unloader valve there. Instead it appears that the unloader valve is part of the head assembly. One video I found showed a later model Quincy with a similar setup, and said the leak in the case of that compressor was into the intake pipe (after the air filter). I'm planning on removing the air filter after pressure builds up (and compressor is unplugged, of course) to see if I can trace any leaks there. The other possibility is elsewhere in the unloader unit on top of the head, but I'm a bit leery of taking that apart without having a rebuild kit in hand (like diaphragms/orings/etc).

Anybody have any experience with older Binks/Quincy reciprocating air compressors?

Thanks for the warning, but I'm not worried about the integrity of the tank. It's been lightly used since around 1980. When I first got this property in 1997, it had probably sat for almost 20 years without use. The first few times I drained the tank, the water had a little rust in it (looked like tea), but once I started draining it regularly, no more rust tint. It's a very well made system (by Quincy, branded Binks).

I was asking here because in order to check some of the potential valve leakages, I'm going to have to disassemble the top of the cylinder head. I would feel better about this if I had a source for replacement parts beforehand. I guess nobody here knows. Thanks anyway!

No, there is a check valve in the assembly at the top of the head.

Nope again.

I'll have to post a photo.

I found a similar setup on the internet, and in that case the leak was found to be from the unloader at the TOP OF THE CYLINDER HEAD. The leaking was into the intake pipe.

On my Binks, the top of the cylinder has an assembly bolted to it, with a protrusion and a 1/4" copper line running down to the oil region of the block, and then from there to between the pressure relief valve and the tank, on the pipe to the relief valve. It's pretty clear that this setup unloads the compression in the head so the machine can start up easier. Of course I could be wrong but when I get around to disassembling the unloader on top of the head, it should become more clear. I'm not entirely sure why the unloader line goes down to the oil region of the block, but perhaps it pressurizes the oil for better lubrication during operation.

Oil pressure? I doubt it. The copper line going from the unloader at the top of the head was leaking some air through the compression fitting, not oil. I tightened all that up and it no longer leaks from there.

I'll have to disassemble it at some point, too busy these days now, and see what's up with it. I still think the internet source I found stating that their very similar compressor was leaking from the unloader on top the head into the intake area is likely what's wrong with this one. And since I can't find any manuals or diagrams for this unit, I'll probably have to take apart the unloader and bring it into a shop where they deal with air compressor issues. It's not a real big deal right now; I'm keeping the compressor off and drained when it's not in use. When I need it, which is rarely, I'll just close off the drain, open the main valve, and use it as needed. I used to leave it activated all the time, but when I heard it cycling on at least once a day I figured it was a waste of energy. I did go around and correct all the little leaks in the piping from the compressor, but when it continued to lose pressure, I closed off the main valve to the piping system and figured the problem is with the compressor itself. I read up on such things and arrived at the conclusion that a leaky unloader or check valve is the culprit. It could also be the pressure switch itself, since there's a pipe from the tank to the switch, but I haven't yet figured out a way to pinpoint that, cause I can't squeeze the leak tester applicator in there. Other systems I researched have the unloader attached to the pressure switch, but not this one.