Compressor tank check valve

Every air compressor with a tank and unloader valve has a compressor tank check valve.

If you have had a chance to read the page on the air compressor unloader valve on this site, you will have already begun to understand why the compressor tank check valve is important to the unloader valve, and to the proper working of your air compressor.

Where is the compressor tank check valve?

Where is the compressor tank check valve on your air compressor? Typically it is located on the compressor tank where the line from the pump head reaches the tank.

Compressor tank check valve

In the photo above the tank check valve is seen within the red circle. The air line coming into the check valve from the left brings air from the pump to the compressor tank. The air line heading up and to the right is plumbed over to the compressor unloader valve.

Your tank check valve may not look exactly like the one in the photo, and, on many of the modern do it yourself type air compressors, the whole pump and control section may be entirely shrouded and you may not even be able to see where the air line from the pump head enters the tank. Still, that is where the compressor tank check valve is normally located, where the line from the pump reaches the tank.

What does the compressor tank check valve do?

Another name for a check valve is a one-way valve, and that’s what the tank check valve does.

I mentioned earlier that it works hand-in-hand with the unloader. The unloader valve periodically opens the line from the pump head to atmosphere for purposes of venting any air trapped over the piston.

If the tank check valve was not in this circuit, when the unloader valve opened, all the air from the compressor tank would bleed out the unloader valve to atmosphere too. The pressure in the tank would drop, the compressor would start, and then it would pump the tank up to the cut out pressure setting, the compressor would stop, the unloader valve would be opened by the pressure switch, and once again, all the air in the tank would bleed out.

The tank check valve prevents this endless cycle by keeping the air in the tank after the compressor pump has pumped it there, even when the unloader valve is open to atmosphere.

What does my compressor tank check valve look like?

Removed from the air compressor your compressor tank check valve could be similar in appearance to the one in the photo below.

Compressor tank check valve

In this typical tank check valve the small, bright male fitting on the left is where the line over to the unloader valve is connected. The part of this check valve above the red thread is inserted into the tank. The air line from the pump head would be connected to the female port not visible at the bottom of this one-way valve.

A non-functioning or poorly working compressor tank check valve is sometimes a problem for air compressor owners. See information on that under the fixing compressor problems tab.

Comments or questions about the air compressor tank check valve are welcome using the form below.


  1. EdwinC says:


    I’m from the Philippines & thank you very much for this great site. I learned a lot from the articles, specially this one about the tank check valve where I finally got convinced that it’s the remaining piece of the puzzle why my compressor is leaking out air not only to the pressure switch unloader valve but I can hear air leaking out too when I press my ear near the cylinder head’s air filter.

    I took a picture of the valve & I suspect the air leaks thru the white silicon/plastic-like stopper ( whatever it’s correct name is 😀 ) back out to the intake & unloader pipes. Originally the surface of this white stopper had a circular impression on it from being continually pressed to the brass hole opening of the valve main unit. I tried smoothing it with a sandpaper but still the leaking issue persists.

    My next move before surrendering & buying a new valve would be either to try putting an o-ring around that opening thick enough to make contact with the white stopper or if I can’t find one that fits I’ll be using that black rubber thing in the picture, or thanks to Juan from Venezuela I will glue a rubber to the white stopper, since rubber is a better sealant.

  2. Hi, my old compressor check valve in dimensions is about 2″, the overall 3 1/8″, is a 3/4″ by 1″, it is assumed that the air flow is 0 – 60, if I use a 3/4 x 3/4 valve the air flow is going to be 0 to 30 cfm, the problem that I have now , is that new check valve is 3 11/16, and I have to install it on a 1 inch tee ,I think the length is going to be way to much for the bottom part , would it diminish the compressor performance or I may damage it? , if I use a 3/4 x 3/4 , otherwise I will have to modify the whole thing installing bigger 1 1/4 components for being able to make it work, please advice thank you

    • A check valve is a check valve is a check valve. If it is installed and plumbed properly, it will work. Your comment about air flow 0-60 concerns me in that I don’t know, for sure, if you are referring to a tank check valve or a pressure switch. Not knowing the make or model of the compressor in question makes it very hard to offer specific advice, Fernando. Please read the pages on this site that explain how the various parts work together to better understand the terminology used here, so that we can be sure that we are talking about the same items on your compressor.

  3. how do i remove broken check Valve that broke off?

    • Jesse, if the tank check valve is broken, then you have threads in the tank I guess? Visit a good hardware store and ask about thread removal tools. They will have a device that will thread into the threads broken off inside the tank port, and allow you to remove them.

      • we got the new check valve on but air seems to be coming out of the part that brother saw piece out

        • Jesse, if, in the process of removing the broken threads the original tank threads were damaged, they will have to be repaired to stop any air leak. That will require, if the damage is not too severe, some specialty leak-stop material. If you Google search for this as I just did, you will find some. You will want to contact the supplier to see if their product will stop the leak based on how much damage was done. Good luck.

    • For anyone with a similar issue, where the valve has broken and the thread remain stuck on the tank. The problem is most likely that the old valve thread has been applied with some sort of glue, not sure if this is just for holding the valve tight or also to provide better sealing.

      In any case I was afraid to damage the thread on the tank, so I have heated the area with a heat gun and then slowly managed to roate the broken piece out of the tank.

  4. Vertical Hausfeld 20 gallon compressor.

    So the problem seems to be the check valve right? Removed the valve to have a closer look and found no flap, no spring, no mechanism to control back flow! I checked in the tank for debris or broken valve pieces and found nothing. Valve is identical to the one in the picture and the one at the beginning of this topic.

    I found a replacement that is identical but cannot get any other pictures of the insides to confirm that there is actually a mechanism. Can anyone confirm that Im not crazy or totally missed something here.

    • That you may be missing part of the old check valve is not the issue we think. The issue is, does the new one work. That will provide you with concrete proof that there actually is a mechanism inside the tank check valve to stop backflow of compressed air. Still, it would be good to see the innards too. We might have to pull one apart to see.

      • I might just match the npt thread size to a check valve that has the flap/mechanism and plumb the rest with a 90deg elbow to get it in the same direction as my inbound air.

        There was an easy start valve that had failed too. (replaced it also) Funny thing…. the unloader and the easy start valve are on the same line. They are both releasing pressure from the pump output line pre check valve. Two pressure releases are better than one lol.

        Would be great if you can confirm the working mechanism in the above check valve. If not no biggie. Thanks so much for your time.

        The specific model is HU502000AV


  5. juan Borregales says:

    Hi everyone, my name is Juan Borregales from Venezuela, I’m trying to build my own air compressor from a fridge compressor, the thing is, I was checking my tank check valve and I realized it was compomised, it was leaking air when I blew it, those kind of valves are very expensive here (like everything else) so I decide to repair it, I din’t open the valve I (it’s the type of valve that are sealed) but you could see the valve wasn’t seating , I took a plastic bottle cap and removed the little plastic (or rubber) from inside, I cutted it with a scissor the same diameter of the top end of the spring, then , pushing the spring down, I placed it inside the valve and now it’s seating, it’s not leaking at all, I put this information if someone need a quick repair of that valve, sorry for my bad English, best regards to all of you

  6. Craig Gibson says:

    I just wanted to thank you for building this site to share all this incredibly helpful info with everyone. Not every DIY guy has the money to throw at new compressors, hours of repairman bill, or even parts. However the better part of us can turn a wrench and McGyverize dang near anything. Haha.

    My Snap-On compressor JUST went down yesterday! It was built by Sleegers, as mentioned in your section about Snap-on compressors. I was having the constant air leakage/drain from the unloader valve. It would stop when the compressor would start up and come back when it shut down. It also wasn’t present if the compressor was unplugged, in the auto position, but below the startup pressure.

    I went through the unloader valve and it was fine. I then read up on this page all about the issues that people have when the check valves fail. I removed mine and found a buildup of nasty rust like crust on the valve (worse then what i am showing in the picture). I sprayed it down with PB Blaster (the only penetrating oil worth buying) in a paint can cap. I worked the plunger up and down and let it sit over night. Low and behold it now works perfect. When i first pulled it out i could blow back through it completely proving it was the culprit. Not anymore… I don’t suggest that though. It didn’t taste any better after its PB Blaster marinade then before. Haha!

    I really put this compressor through its paces though… I ran my plasma cutter and various air tools on it consistently through the entire engine swap project on my 2007 Kia Sorento. It went from 3.8L V6 to an American monster… A 6.0L LSx V8! Sleeper is an understatement!

    Thanks again for taking the time to build this site!!

    • You are very welcome. We are glad the information helped. Good luck with your Sleeper. Rubber side down, right!

  7. I have a 20 Gal Husky 947265. Is there supposed to be a pin hole in the cap of my check valve. The tiny plug that unscrews has a pin hole in it. If I run the compressor, then unplug it, air leaks from a pin hole on this plug of my check valve. It is a continuous leak. Here’s the part (Part Number: E106123).

    • When we view the part number in question, we see no pin hole in the cap of the check valve. However, even if there is a hole, the check valve should not allow air to flow back out of the tank to atmosphere. Once air is compressed into the tank the only way out should be through the discharge pipe, regulator, and discharge coupler. The valve in the check valve is not seating properly if air leaks out. You can try cleaning it first, but given the hole, maybe it’s time for a new one? Lots can be found on line.

    • I have the exact same model and exact same problem. Air leaks from the pin hole on the cap. The compressor will not fill the tank and only bleeds are thru this valve cap. I am wondering if ordering the whole repair kit will fix this. I took my valve apart and cleaned it but that did not fix the leak at all.

      • John, this is a smaller compressor with a small, insert into tank, pressure switch. There is no room for a traditional unloader valve. The pin hole in your tank check valve is there to allow air from over the piston to bleed out when the compressor stops. It’s also bleeding air all the time the compressor is running, but the pump capacity far exceeds that bleed volume, and the tank will fill to cut out as it is designed to do. If air is leaking from this hole when the compressor is stopped, it’s the tank check valve that is the culprit. If your tank pressure never gets to cut out, and the compressor runs and runs, that’s typical of a gasket, valve or piston ring failing.

  8. Erin Montgomery says:

    I just took my check valve out, and it had broken a coil off of the spring and jammed the piston/seal out of whack. Took it apart, stretched the spring enough to hold tension against the seal, cleaned it up, and reinstalled. Good as new!.

  9. Ran across this article after removing my check valve because it had a small leak at the threads where it screws into the tank. It was a bear to get out, threads stripped coming out. Glad it was aluminum, so it did not damage the manifold it was screwed into. When I got it out, I noticed that both snap rings that hold the one way valve into the lower part of the housing had came out of their ring grooves and the valve was dangling in the housing. Ready to be replaced after 33 years.

  10. Hi Willy,

    Thank you very much for your information, After I read your website, it took me 2 minutes to locate and clean my check valve (Had to get anew one because the old one was damaged.)
    Great site, very informative.

  11. Just ran into the same problem of air escaping from the unloader valve once the tank was up to max and the motor stopped. I opened the check valve and there was a small piece of rubber sealant holding the check valve open. Almost went to buy a new compressor. Thanks for the tip.

  12. My unit is a MAC2400 and is approx 10 yrs old. After reaching 120 psi it shuts off and leaks down to approx 80 psi at which time it stops leaking. Occasionally, after a few cycles, it stops leaking and performs normally.

    I have started disassembly with the thought of replacing the check valve (which is difficult to get to). From some research it seems the check valve is a common failure, but there is the question of whether it might be the unloader valve. I regret that I did not find this site before starting disassembly.

    Can you offer advice?

    • We think that with a tank pressure above 80 PSI your tank check valve leaks, and below that, it doesn’t. Could be debris in the seals, worn tank check valve…? We would re & re the tank check valve in an effort to repair the compressor problem.

  13. marcpilot1 says:

    When combining two air compressors, for instance a 26 gallon one and a 21 gallon one, it says to install a check valve at both the compressors lines. 1) Does anyone know what that valve may look like or what it is versus other little ball valves I have seen for air compressor lines at the stores, and 2) where they want these check valves installed into the two compressor integrated system.

    I would think they would go right there at the compressor, like before the line leaving the compressors and going to the tee. The tee meaning I am assuming a line leaves both compressors and go into a tee, then a main line leaves the tee and runs your tools.

    As you can tell, I’m not too expert at setting up a 2 compressor system but I have a 3′ lead hose leaving both compressors and going into the tee, then a 25′ line going to a double filter thing for catching moisture, and at that point, another 25′ hose going to the hvlp gun or any other tools I need to run (impact wrench, die grinders, etc, etc) But I am really concerned about making sure my homemade system is as correct as I can get it for the painting of auto body parts and car fenders, hoods, whole car, etc, etc.

    Thanks for any help. Btw, reason for all this is cant afford a 60 gallon or anything but I do have a gently used 21 gal from HF and newish 26 gal Bostich from WM.

    THANKS again, very much so, for any suggestions, guidance, advice, etc!!

    • The two check valves on the output lines would likely serve to balance the pressure if the tanks are at a different starting psi. The tank with the higher starting pressure would be used until it matched the tank with the lower starting pressure. Both tanks would be used equally from that point on.
      A potential issue i can see with this approach is that one tank/compressor may never reach the pressure switch cut-in point. if the other one runs first because it`ll always take precedence being at the higher pressure.

      • Absolutely right. If the one air compressor can deliver enough air for the air tool, the pressure in the second compressor tank may not fall far enough to make the compressor cut in. But, that’s the idea, isn’t it? You only want the second air compressor to cut in if the first cannot deliver enough flow. Those with the know how and that have compressors with adjustable pressure switches can alter the settings to make the secondary compressor cut in sooner, if desired.

  14. Excellent website, very clear and complete descriptions. My unloader valve leaks after the pump stops so I think this is the check valve. I will disassemble and try to clean the check valve, I assume clean with wd 40 ? Thank you for such good descriptions….

    • You are welcome, Jack. If your check valve has rubber seals, nope, we wouldn’t use WD as it tends to enlarge and soften rubber or Buna N seals. If really hot water and soap doesn’t do the trick, paint thinner or varsol would work we think.

  15. Felix Oquendo says:

    Just bought a used compressor. It has the check valve with an unloader valve. The unloader valve has a fitting where a small line is connected. However, the other end of this line is open or not connected to anything…it bleeds to the atmosphere. When the motor is running, there’s air bleeding out of this line continuously. Is this the way the unloader valve is supposed to work or is it defective? It doesn’t bleed when the motor stops as the predetermined pressure has been reached, so I know the check valve works fine.

    • Some lower cost air compressors have a continuous bleed of air out of a hole while the compressor is running. This is a cheap way of eliminating the need for an unloader valve, as the air over the piston is escaping all the time the compressor is running, and then the bleed stops after the compressor reaches cut out, and all the air over the piston is gone. Unless, as you point out, the tank check valve leaks. We don’t know your compressor make or model, so we can’t be any more specific than that.

  16. I think most air compressors have a compressor tank check valve manufactured by a third party.

    Companies tend to specialize anymore, I believe.

    Does your post ask why the screw cap on your compressor check valve fractured? If so, the reason I think is that in the drive to offer lower and lower prices for the DIY type air compressors, compressor assemblers tend to acquire parts from the cheapest source. A cheap source may not have all the checks and balances in their quality control and manufacturing processes to weed out the the poorly built parts, and it sounds like you, my friend, got a piece of crap on your air compressor.

  17. Just reading what you said “The unloader valve is open when the compressor is off” which implies it should be closed when the compressor is on ? But mine appears to be open permanently ? However, like I said, it works fine with the belt pump.

    I will see if I can contact someone who has built one, I think. Maybe I am missing something !!

  18. I assume the unloader only works, when the compressor power is cut off by the pressure switch. The compressor turns off, closing the check valve and diverting unused air to atmosphere ? If I blocked the route to atmosphere with a blanker, would that a problem do you think ? Not sure how a fridge compressor works either.
    I am a complete novice as you can tell !!! But I am an engineer so not a complete idiot !!! Just a slight one !!

  19. Hi WillyR,

    Thanks for replying. Thats what I was thinking about fridges !! They dont have one in the system normally, so can I modify the tank set up, so the unloader is not used, then do away with the check valve (or add an inline one to prevent back pressure) ?
    Do you think that would be OK ?

    • Have a look at the unloader valve page to better understand what it does. Yes, and air compressor typically requires a working unloader in order to help in the motor restart on cut in.

  20. Hi, I am trying to build a silent compressor using a fridge pump and a 6ltr tank from a belt driven pump. My problem is that the check valve wont open to fill the tank ? Is this purely down to the fridge pump not being big enough ? Is there a solution, other than getting a larger more powerful unit ?

    • Hi Wayne. The purpose of the tank check valve is to keep the compressed air in the tank when the compressor is stopped, as the unloader valve is open when the compressor is off, and the air path from the pump head is open to atmosphere. If the check valve wasn’t there, then the tank would empty every time the compressor was stopped and the unloader valve opened. So, I suspect that the check valve you are using may be in backwards, and that would prevent the compressor pump from driving the air down into it. Don’t know much about fridge compressors, though I suspect they do not have an unloader valve, else the refrigerant would exhaust every time the compressor stopped.

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