Air Compressor Draining Tank Thru Unloader Valve

[Leo Butler]

Good morning! I’m trying to diagnose an issue with my old Craftsman air compressor that recently started. It pumps up the tank just fine, but as soon as the motor cuts off, the tank drains completely through the unloader valve. I thought the problem was with the rubber diaphragm in the pressure switch, but after replacing the entire switch the problem persists. I figured that meant a reed valve in the cylinder head had broken, but I removed it and checked/cleaned all of them and they seem fine. But I feel sure that must be where the problem lies.

I removed the line from the unloader valve and allowed the tank to pressurized to about 20 pounds then turned it off; pulled my finger away and it really moves some air. I still can’t see how it’s not a bent/broken reed so I’m going to pull the head again to take a look.

Anyone have any suggestions for what to look for? Cracked head or cylinder maybe?

Thanks!

[Bill Dufour]

Unloader valve? Do you mean the schrader valve on the pressure switch or does your pump spin all the time? single or two stage?
I would suspect the tank check valve. It may just need cleaning.
Bill D

[Ken Combs]

The check valve between the pump and tank is bad. The unloader bleeds off the pressure in that line. The check valve is supposed to retain the tank pressure, but is allowing it to flow back toward the pump, thus out the unloader. I think all the old Craftsman had the valve screwed directly into the tank, then the line attaches to it.

[Sebastien La Madeleine]

I have had similar problems in the past. The air filter on my compressor was old and ended up shedding a piece of media the size of a needle head that went through the compressor and stuck to the oil film on the ball of the tank valve, preventing the seal of the tank. The unloader would drain the whole tank through that leaking seal. Just a bit of degraser to clean the ball from the tank valve and the problem went away. That was almost a decade ago and the problem never came back. Granted I change my filters more often now.

[Leo Butler]

The check valve between the pump and tank is bad. The unloader bleeds off the pressure in that line. The check valve is supposed to retain the tank pressure, but is allowing it to flow back toward the pump, thus out the unloader. I think all the old Craftsman had the valve screwed directly into the tank, then the line attaches to it.


I have had similar problems in the past. The air filter on my compressor was old and ended up shedding a piece of media the size of a needle head that went through the compressor and stuck to the oil film on the ball of the tank valve, preventing the seal of the tank. The unloader would drain the whole tank through that leaking seal. Just a bit of degraser to clean the ball from the tank valve and the problem went away. That was almost a decade ago and the problem never came back. Granted I change my filters more often now.
Guys, this sounds exactly right, but I'll be darned if I can spot anything that looks like it could be a check valve on that pressure line. See this pic; I've removed the bigger line on both ends and the brass fittings are straight thru, no ball in sight. Could one of these fittings be a check valve, and it wore so badly that the ball blasted thru into the tank?

493600

Thanks for the quick replies, much appreciated!

[Sebastien La Madeleine]

In my case it was the fitting from the big line to tank, the check valve itself was inside the tank. As I unscrewed the fitting the second brass part came out and it was there!

Nothing is impossible maybe it was badly manufactured and the check valve part fell in the tank.

[Leo Butler]

ZOMG, it's been proven once again - if you know what to search for, someone has documented your exact problem on Youtube (https://www.youtube.com/watch?v=0uWs9x2uhDU). This is even the smaller version of my compressor, looks exactly the same!

Looks like I'm one functioning check valve away from getting it back in service. My brother gave it to me, oh, 25 years ago after he seized up the compressor by running it out of oil. I got it freed up and tightened up all of the bolts and it has just seeped slowly since then, been a real workhorse. I clearly didn't need to replace that pressure switch, but because I had to swap the old base with the male fitting onto the new switch (Chinese-made, but having disassembled both I can say it's a 99% identical clone of the existing one) I'm an expert in doing that now. :)