Air is leaking at the pressure switch end of compressor tube?

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I have a DEVILBISS upright 5.5 hp. Model No. FB5520VP. When shutting off at cut-out pressure, or when manually turned off, air continues to escape at the right angle connection of the pressure relief tube and the pressure switch assembly. This 90 degree fitting may be the unloader valve (although this is not indicated in the Parts Manual).

Should / Can this valve be replaced separately, or does one have to replace the entire pressure switch assembly? This question comes in from Reggie Griffith of Jonestown, Texas.

Unloader valve on side of pressure switch
While air may leak out here when the compressor stops, the issue is often a leaking tank check valve!

G’day Reggie.

I believe that you’ve got a check valve issue and not a valve at the pressure switch issues. That valve you speak of is, I believe, the unloader valve. I’ve uploaded an image of one above, and hope this is what you are talking about.

The troubleshooting pages lay out steps to deal with this problem, yet simply, dump all the air with the compressor off, remove, and check the tank check valve.

Let us know if this helped or is the problem resolved?

Another Pressure switch leak

Inside the electric black box on compressor is a leak of air, there is a little 1/4 inch air hose that comes off the tank and I think it shuts the compressor off when it reaches max presser. There is a small needle on the end of it that makes contact with a part in the box, but when it reaches max and shuts off it leaks and keeps on leaking till all the air is gone? ( just started to do this has been working fine for a year or so) Is there a way to fix this or is it a bad switch? Is there a diagram to this to show how it is supposed to work?

thank you Mark Meyering, of Hastings Michigan.

Mark, what you are describing is a common occurrence in many compressors.

That little valve you are referring to, the one beside and operated by the pressure switch toggle, is the unloader valve.

When your air system reaches cut out pressure, the pressure switch reacts, opens the circuit to cut power to the motor, and at the same time, the lever on the side of the switch toggles the unloader valve.

This opens the air line from the top of the piston to atmosphere, and allowing all the air trapped in the line and over the piston to bleed off.

Once this air is gone, the unloader valve, though still open, bleeds off no more air. Except yours still bleeds air.

When the pressure in the system reaches cut in, the pressure switch trips again, and this releases the pin on the unloader valve allowing it to shut, and for the air that is now being compressed, to flow down the line, and into the tank, past a check valve that is located at the tank.

If, however, your check valve hasn’t seated properly, or there is debris in it, or it has failed, it doesn’t close when the air flowing into the tanks tops because the compressor motor stops, and air flows back out of the tank.

Where does it go? Right out the unloader valve. This bleed will continue until the pressure turns on again, the unloader valve will close, and air pressure in the tank will increase to cut out pressure.

At cut out pressure the unloader valve opens to bleed air over the piston, but now it’s also bleeding all the air back out of your tank, as the check valve, that is supposed to keep the air in your tank, isn’t.

See Check Valves and Unloader Valves linked from the site map or the Valves page.

Shut off the compressor, dump all the air, and pull the check valve assembly for inspection and cleaning, or replacement. This should solve your problem.

And again, Unloader valve continues to vent air.

by Terry

The unloader valve continues to vent air. It used to work perfectly, but now there seems to be a problem with some sort of internal “butterfly” valve inside the compressor tank that continues to allow air to bleed back from the tank through the supply line and then out through the unloader valve, from Terry of Oakdale.

Terry, I don’t know about any butterfly valve, but I do know that you need a working check valve.

See the CHECK VALVE page for details on cleaning or replacing yours, as I suspect that’s the issue with your compressor.

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Looking for some help here… 😉 I’ve got an older Devilbiss Oil Free Air Compressor (Model IR350E20AD)… at least 20 yrs old or more, but has always been reliable as heck. Suddenly started leaking air the other day around the pressure switch after tank was filled. Read the related threads and agree… it’s leaking from the tiny ports on the Unloader valve, and only happens when it shuts off, or i turn it off with the toggle switch… so i assumed it was the check valve. Problem is, I can’t find any check valve and the exploded parts diagram from… Read more »

Your pix didn’t make it. Not your fault. You might try again.
You certainly don’t need to toss it.
Still, if you’re talking about the items below either item 26 or 34 depending on type, there’s a fair chance you can either clean them up or improvise the bad parts. I’m guessing it’s some rubber piece in there causing the leak.

If that fails, you can get some other check valve and jerry-rig it up.
You don’t don’t need to follow form, just function, in pipe or what-have-you.
Don’t forget to use plumbing tape either way.

good luck

Re: the pic cannot be displayed because it contains errors×279.gif

…and apparently attachments is broken again. Not indication uploaded .png got there.

I have a check valve problem, just put 2 new pressure switches on and still leaks, why?

Last edited 1 year ago by Bill Wade

Sounds like you’re unclear on the concept. The check valve is usually located on or very near the tank. The pressure switch is usually where the *unloader valve* is located, but not the *cause* of the leak, even though that’s where the air comes out.
You need to repair or replace the check valve. Follow the (typically) 1/4 inch tube from the pressure switch toward the tank. The thing it goes to is the check valve, and what is actually leaking. You can remove and very possibly repair it, or just replace it.

Good luck.

I figured it out, put a new check valve in and all is well, didn’t realize it would back flow through the pressure switch

Thanks for the update – glad you got it sorted.

Had to replace pressure valve and unloader. Pressure valve is a 4 port type. Since installing new parts I have had 3 occasions of the unloader tube rupturing during initial start up. Pressure switch is 100-130 psi. Unloader tube rating is max 225 psi. Cannot find any info on the unloader tube rupturing. Any help / advice will be appreciated.

Compressor make and model and maximum output pressure?

US General, model US660V 3848, 60 Gal, 3.5 HP. Max pressure 130 PSI.

Very odd given that the unloader tube is void of air when the compressor is off, and only sees pressure when the pump is running. The material of the unloader is plastic? Is it really old? Can’t figure out why this would occur and it’s not something I’ve heard before.

I may have mis-communicated symptom. I replaced pressure switch assembly which included a new unloader. During install, I also replaced unloader tubing with 1/4″ tubing that is rated at max pressure of 225 PSI. Turned compressor on. It filled to 120PSI at which time the new unloader tube ruptured. The air leak from the rupture was major (sounded like an open line). The leaking rupture in the unloader tube stopped as soon as I turned compressor off. Removed the ruptured unloader line and replaced it with different brand tubing of same size and rating. same result. Any chance the tank… Read more »

Larry, when you say rupture, you mean it splits lengthwise? Or something else?
Can you post a pic of what gives?
Are you certain the OD of the tube is correct for your fitting?
Anyway, nice machine. I was gonna buy one myself, then they stopped carrying it. It’s Bel-Aire / ABAC.

Rupture of tube looks like a blow out. Unfortunately do not have a picture as I trashed damaged tubing. Blow out was about 2 inches from unloader fitting on check valve assembly. Location was consistent for each failure. Tubing size (1/4 “) was correct replacement. May have made progress yesterday. I removed tank check valve, cleaned it, and reinstalled. Also removed automatic drain valve which receives air from same line as unloader. My thought here was to eliminate as much as possible. This returned compressor to configuration it had when installed some years back. Used same tubing for unloader. Powered… Read more »

Thank you for that clear explanation. Good plan.

Maybe you can send pix before and after you re-install the automatic drain (is it the HFT unit?), if it holds up the next day or three…

It’s a mystery to me why the line would blow, unless it’s some odd characteristic of the plastic where it can’t take a *sudden increase* in pressure, maybe somehow caused by the auto-drain.

Hi, I have been somewhat remiss on update to this site. After removing auto-drain device, all seemed well until last Friday. Then blew another unloader tube. Replaced it this AM and all seems well again. Will get a couple photos and attach to next update.

Hi again. Have attached couple photos of unloader tube that failed earlier this week. Hard to get good detail due to tube being black. Damage is consistent with previous failures.

I think it’s time to check the temperature derating on that tube (or tubes).
It’s blowing at the check valve, right? Maybe you can check the temperature there, and see how it looks vis-a-vis the the derating graph, assuming you can find one.

Thanks for a great idea. Will try to check temp at check valve area. I know it gets hot there, but do not know temp for sure. Do not have derating chart or this or other tubing. Not sure where to get one from. Might try to fab a heat shield for tubing in area of check valve.

More like a radiator, I’d think…

You’ll probably be able to find a chart if you look at a commercial supplier or manufacturer. The tube label may enable a cross-ref, if not lead directly to a chart.

Oh, here’s one – the derating is pretty spectacular!

Last edited 2 years ago by Doug in