Air continuously leaks out of the unloader valve

Air continuously leaks out of the unloader valve on your air compressor, and this is a problem…. why?

Well, if air continuously leaks out of the unloader valve, then the compressor tank is emptying, and the compressor needs to restart to replenish the air in the tank to get back up to cut out pressure so it can stop on its own.  But then, before you get to use any air, it’s all leaked out again, and the compressor restarts to fill the tank again.

Or, in some cases, compressed air can be bleeding out as fast as the compressor can pump it, and this leads to overheating of the compressor motor and other issues you would rather not have to deal with.

First though… where is the unloader valve.

Where is the unloader valve?

Let’s identify where the unloader valve is on a typical reciprocating air compressor. The unloader valve might be inside of, or attached to the outside of, the compressor pressure switch.

Photo of two air compressor pressure switches

In the photo above are two popular compressor pressures switches. You cannot see the unloader valve on the Condor switch on the left as it is internal, and the air line to it is plumbed into the bottom of the switch.

You can see where the unloader valve and air line from the pump head connects to the Lefoo switch on the right in the photo above. The brass fitting on the left side of the Lefoo switch is the air line connection, and under it is the unloader valve.

 Where is the air leak?

Since the Condor pressure switch has an internal unloader, if air is leaking from it, it may be blowing out any opening in the switch cover.

On the external unloader, you can feel the leaking air around the unloader valve itself.

Why is compressed air leaking out of the unloader valve?

If you have read the page providing detailed information about the unloader valve workings found under Air Compressor Information in the right navigation bar, you will know how the unloader valve functions. No need to repeat all that information here, except to say that if the unloader valve is working properly there will be a burst of air from it (1-2 seconds at most) when the air compressor reaches cut out and stops. After that air should stop blowing out of the unloader valve or pressure switch.

If air continuously leaks out of the unloader valve when the air compressor is running, that indicates that it is the unloader valve itself that has failed, and it needs cleaning and / or replacing. If you cannot resolve the leak by cleaning the unloader valve (which may be more trouble than it’s worth to attempt) then replace the pressure switch and that should include a new unloader valve too.

Is Air Coming Out the Check Valve?

When the air compressor shuts down and unloads, then all the air that may have been trapped over the piston is now vented by the unloader valve. It’s gone. That means that if  compressed air continuously leaks out of the unloader valve, the  only other place where that compressed air might be coming from is the air tank itself.

What is supposed to keep the air in the compressor tank when the unloader valve is open? The tank check valve. (Detailed information on the tank check valve is found under Air Compressor Information in the right navigation bar).

If air continuously leaks out of the unloader valve when the compressor is not running, then your compressor tank check valve has either failed completely, or the internal seal has not seated properly and this is allowing air from the tank to blow by that seat and escape out the unloader valve which, when the compressor is stopped, is open to atmosphere.

How to fix it?

In order to stop the air leak out of the unloader valve you will want to…

1) Unplug the compressor

2) Drain all the air from the tank

3) Remove the lines connected to the tank check valve

4) Remove the tank check valve, inspect, clean, re-install the same one, or install a new one

5) Reassemble and test the compressor operation. If the check valve is now working, once the compressor unloads at high pressure cut out, air should stop leaking from the unloader valve.

Where do I get a new one?

Even though parts supply for the typical home or small workshop air compressor is always a problem – not too many people that sell this type of air compressor stock parts – you can replace your tank check valve with any vendors tank check valve that fits the thread in the tank and to which you can attach the line to the unloader valve.

Questions about air continuously leaks out of the unloader valve are welcome using the form below.


  1. BK, None of the images you attempted to provide appeared, nor does your post. Sorry. In any case, is this a gasoline operated air compressor? Typically, air bleeding out of the unloader valve all of the time points to a leaking tank check valve.

  2. Anyone know what type of compression fitting this is ???

  3. Hello,
    I have an old Sears 2 stage five horse compressor that was kind of a basket case when I got it. I converted it to single phase and it builds full pressure but seems to take a long time to get there. I am not sure if all the parts are there. In the picture the valve at the top of the primary cylinder always leaks while the compressor is running. The line that goes to the front of the crankcase runs to the secondary cylinder valve. Those are the only two unloader type valves I see. Should the one on the primary cylinder always be leaking? Or is it a burst disk type that has popped and needs to be replaced?

    • I am sorry, I don’t see the valve in the photo you uploaded. If it is a dual cylinder, sometimes the primary will have a PRV on it so, in the event the intake valve in the secondary cylinder fails and air cannot flow into it from the first, the air will vent out the PRV rather than build unsafe pressures in the line. If you can remove that valve and examine / clean it, that may help.

    • that unit may have a rigid piston, and the head has warped, ether, may have dried er out 2,

  4. Hello there.

    wonder if you can help. I have a small compressor that Ive converted to a silent compressor by replacing the old motor/piston with a fridge compressor. My problem is that when first started everything works fine. Compressor runs gets up to 170 cuts out. I then get a continuious hiss of air from the stock regulator/cut off valve, also compressor may take a minuite or 2 to start( I’m told that the pressure in the compressor must equalise before it can start). I’ve been told that the hiss is caused by safety valve built into the switch because the pressure from the compressor is not as strong as the original. Also that I should add an uploader & check valve which should help will the delayed start. Could I please have your thoughts. The shut off switch is a stock 4 port that came with compressor(common on ebay).

    Great site

    Many thanks


    • Neat. Cause of course, that’s what many silent compressors are, compressors with a refrigerator type compressor pump, often mounted on springs so there is no vibration. Good on you for taking the step towards a silent compressor. No idea what HP your compressor motor is, yet if it is the same HP as the refrigerator motor, there shouldn’t be a problem. A significant difference between refrigerator pumps and compressors though, is that the refrigerator is a closed loop, while a compressor typically is not. When the compressor stops at cut out, on a typical compressor, an unloader valve opens to vent air from over the pump piston, to allow easier start. So, did you install an unloader valve to vent the air over the piston? Often the pressure switch may come with one, but you need to plumb it.

      About the “continuous hiss of air” ask yourself, where is it coming from? The tank I expect, which would suggest that your tank check valve is not working. Please read the pages on this site that talk about the various compressor components and how they work together for more info.

  5. Steve schwenke says:

    I have coleman 5hp 27 gallon air cooled compressor. Air starts leaking out of unloader valve continuously while pump is running from 40 psi till it pump shuts off at 125 psi and then its still leaking air out. Would definitely think that i have a problem with check valve since its leaking out after pump cuts off but would the unloader valve also be bad since its leaking out while pump is running

    • Sure sounds like the unloader valve isn’t cycling properly. It should be shut while the compressor is running, and open when the compressor is stopped.

  6. This valve leaks as soon as I turn on the unit….it is brand new, what gives??

    • If you are speaking of the Unloader Valve leaking as soon as your compressor (would be nice to know the make and model) is turned on, that suggests the unloader is not closing when the pressure switch trips to passing power to the motor. If I’d bought this compressor, I’d take it back for a replacement.

  7. I have an IR 2340L5-V compressor. I have replaced the in-tank vertical check valve (~$50/ea) several times in the last two years. It has now failed again. What is causing this part to fail so often? There is no auto drain, but i do drain the tank weekly.

    • When you say it’s failed, I am assuming it is leaking? If this is the check valve where the air from the pump enters the tank, I’m wondering if – instead of it being something in the tank – if your pump cylinders seals are worn to the point where more than a normal amount of oil is bypassing them, and entering the tank through the tank check valve. If that’s happening, and you have a dusty air getting through the intake filter, the combination may be enough to foul the seals on the tank check valve.

  8. Air leaks through unloader muffler assembly on my 3z182F champion compressor.

    • Randy, I don’t know of any muffler assembly on a 3Z182F pump. If what I think you are referring to is the intake filter, then that points to an intake valve leaking.

  9. ron savoie says:

    I have a 60 gallon compressor with a 5 hp motor

    i just replaced the pump with a new 3-hp single stage air compressor pump

    on the old pump there was two lines one going to the compressor tank check valve and the smaller one going to the unloader valve next to the pressure switch

    the new pump only as the one line going to the tank pressure release valve so i used a tee fitting on the tank itself and reduced it to 1/8 npt and ran a line from there to the unloader valve

    now tonight i wired it to the panel and started it up it ran till it got to about 90 lbs then i turned it off and it was leaking from the unloader valve

    now is this because i am running the line directly from the tank is why it is leaking or do i have to find another way to run the line from the pump head . thanks..

    • If the air is coming out of the unloader valve when the compressor is not running, that points to a leaking tank check valve. Make sure yours is not leaking, and if the problem persists after that, add a comment here.

      We also wonder why it only ran to 90 PSI, or did you just decide to stop it?

  10. Hello, I have a 29 gal central pneumatic compressor that leaks air out of the unloader only while it runs. After the motor cuts out it it will continue to release air for a few seconds and then closes as it should. Is this normal? Thank you for your assistance.

    • No, it is not normal. We suspect your compressor unloader valve has failed. It is supposed to be closed – not allowing air out – when the compressor is running, and then, when the air compressor stops, the unloader valve should open to vent air over the piston. After that is complete, the valve stays open, yet air stays in the tank due to the tank check valve. When the compressor starts, the unloader valve should close again.

  11. Is there a reason for unloader valve failure? Is there any disadvantage to having the unloader valve in the check valve vs. in the regulator?
    I recently had to install a new pump on my unit. I had to change from 1/2″ to 3/4″ check valve and the one the dealer had had internal check valve. It failed after a week, kept bleeding off air from the tank but not off head of compressor, leading to compressor lockup.
    I replaced it with a new one. Since then, it had failed twice more, this time I simple cleaned and reinstalled, a real pain. The last time though, I was out of the shop, the over temp failed and it appears the motor burnt up trying to start. My old unloader valve, located on the regulator worked for years with never a failure so I am thinking of changing the check valve and going back to that.

    • We don’t see that you have identified the compressor make or model? We don’t normally see an unloader valve with the tank check valve. Typically the unloader valve is part of the pressure switch, either attached to the side of it, or one with an internal unloader valve. That whole scenario may depend entirely to what compressor you are referring?

      As this comment “My old unloader valve, located on the regulator” we wonder if by regulator you mean pressure switch?

  12. Hello

    I’ve got a Craftsman Model #92116472 1.6 HP 33gal. Compressor and the transfer tube broke at the check valve nut. Purchased a new check valve (part #031-0094 ), nut (for 1/4″OD Part #058-0017 ), and bleeder tube (aluminum air transfer line part #145-0626) and figured I’d just replace everything since the cost to do so was minimum and the unit is 3 years old and this is the first time we’ve had trouble of any kind with this compressor. My question is, how do you remove the bleeder tube from bleeder valve (part #136-0090)? It appeared as being some type of quick disconnect fitting, as there appears to be nothing retaining the air line in the swivel fitting. I’ve applied pressure downward on the collar of the fitting typical to how you’d remove air transfer tubes from male quick disconnects, but for the life of me I can’t figure out how to remove that bleeder tube from the bleeder valve, any help would be much appreciated, and thanks in advance.

    • We think that the switch we refer to is this one:

      Condor pressure switch

      That being the case, the tube into the bottom of the switch is 6 mm O.D., and it is a quick connect fitting into which the tube fits. You are on the right track in pushing down on the collet as you try to remove the tube. It also helps to push in on the tube as you push in on the collet, before you try to pull the tube out. Good luck with it.

  13. paul fleming says:

    I disconnected the copper tube at the pressure switch that comes from the check valve . Then attached another copper tube to that tube and ran it down to a Kingston auto tank drain .To the end of the pipe from under the tank , i attached a “tee” and put a ball valve on the left & the Kingston on the right .
    When the compressor completes its cycle , there is a very light hissing from the Kingston outlet . If I let a small amount of air out of the tank via the ball valve , with the electric turned off , and then turn the compressor on , immediately i hear the Kingston close while the tank is refilling . After the cycle is complete , the hissing is back .
    Can you tell me what i’m describing ? My friend owns this 80 gal compressor & claims it’s only 4 months old . When he told me that , I didn’t bother to inspect the check valve .

    • We tend to think the the Kingston Auto Drain is the problem. It should reset when the cut out occurs, and auto-dump the tank water when that happens. That air bleeds out of the drain valve all the time, that suggests that the inner valve has been compromised somehow, and that it’s not sealing correctly. Since we do not have one of these tank valves to test, we suggest that you forward a link to this page to: and they will provide a response.

  14. Condor switch on air compressor is in off position and machine still turns on. My husband is not home and over the phone says it is impossible that it would go on in the off position. This has happened before and I am looking at it! It is in the off position. Can anyone help me win this argument! Please and thank you!

    • If the points inside the switch are fused together, or if the toggle that flips the switch to OFF have failed, then yes, your air compressor can run even though the switch is toggled to OFF. 🙂

  15. This is the Best and most thorough site I have ever found regarding air compressors and their repair.
    Many thanks to the authors for taking so much time to assemble this site and sharing it.

  16. hi, after changing the motor on the compressor, the unloader release air once i turn on the compressor. The air never get into the tank, but all release by unloader valve. It was working finr before i change the motor. Does it caused by changing the motor from 3 horse power to 1 horse power?

    • Howdy. As long as the compressor pump works with a smaller HP electric motor, you should be good to go. That air continuously leaks out of the unloader valve tells us that it is likely your tank check valve that is compromised somehow. Dump all the air with the compressor off, pull and check the tank check valve. See the page on this site about tank check valves for more information.

      • Willyr;
        Thanks for super quick reply.
        I try took out the check valve spring and the rubber to check if it is the check valve.
        The check valve spring work fine, i close the check valve without including the spring and the “rubber” to see it is the check valve issue.
        Still the air go directly to the unloader valve and no air go to the tank at all.

        • If the air in the tank cannot escape via the tank check valve and get back to and out of the unloader valve, which is open all the time the compressor is off, then it must be the unloader valve itself that has failed open. Changing the motor shouldn’t have affected the unloader valve, though when wiring it you were likely into the pressure switch, and if your unloader valve is part of the switch then you may have affected the operation of the valve. With the tank empty of air, turn on the compressor, and watch the unloader valve if it is external to see if the lever moves the pin. If the unloader valve is internal to the pressure switch, then it may be time for a new switch, if the unloader will not stop leaking.

  17. I have a sears 5 h.p 25 gallon air compressor. The compressor runs fine but I cant build up pressure in the tank. I have changed out the check valve, the unloader valve, the regulator and rebuilt piston kit I still have the same problem it runs and runs but never builds up pressure? what next?

  18. Hi, i am having problems!! my compressor fires up fine, shuts of fine, but then air continually leaks out of i think the uploader. i can nudge up a little bar and that shuts it off. So i bought a complete new pressor switch and it does the exact same thing.

    please help James

    • James, we hope you kept the old pressure switch as a spare. We believe there is nothing wrong with it. If air is continuously leaking out of the unloader valve it’s usually because your tank check valve is leaking. Please have a read of what the unloader valve does and what the tank check valve does to better understand how the two work together. You’ll want to power off the compressor, drain all the air from the tank, locate where the line from the pump head reaches the tank, and there is typically where the tank check valve is. Remove it, clean it, test it (air should only pass through it one way) and then if cleaning it works reinstall it, or replace it if cleaning doesn’t resolve the issue.

      • thanks buddy, spent days now screwing with the switch :-)) lucky i am the boss. i will check the tank check valve tomorrow.

        • Hi Wille, so that totally worked, thank you so much.

          but of course now i have a new problem, at first everything was fine, it shut off at 10bar and kicked on again at 7bar. but then every second time it would not kick on unless i pressed the ON switch. now it is not kicking on at all unless i press the ON switch.

          when i had the problem yesterday, i kept fiddling with the adjusting nuts for the shut off and kick on.

          thanks again for your help, James

          • If it is the ON/Off switch on the pressure switch, it’s likely the pressure switch itself is the problem. If it’s the thermal overload switch you have to keep pressing, first off, stop pressing it, as you are overheating the motor. If the motor is going off on thermal overload (may just be that you’re running it too long??) and it’s not from over use, then it’s time to start again. Provide details, please.

  19. I found this via Google search and it just saved my compressor. My airbrush compressor HaoSheng AS196 (the same applies to AS189) leaks big time through the unloader valve and doesn’t restart until the tank pressure has dropped to almost atmospheric level. Meanwhile, the motor coil keeps buzzing and heating up. I initially thought it was the unloader valve (because it leaks air), then the pressure sensor (because it won’t restart). Lucky I found this article. I obtained a new check valve rubber cap from the manufacturer, replaced the stiff old one, and the compressor works like new now!!!

  20. I have a central pneumatic 6 gallon/150psi air compressor. I don’t use it much but wanted one just in case. In the few times I’ve used it the compressor fills as normal but when it hits the 150lb mark the relief valve pops & releases the air. I can push it back down & it will usually not pop again til it needs to refill. Is the PRV the problem? Thanks

    • Eddie, see the new page just created – Compressor PRV Bleeds Air – in response to your question. If that doesn’t provide the answer for you, leave a comment on the new page.

  21. I just bought a compressor that is kinda home made/ parted together and while getting it setup I managed to bump the unloader valve hard enough to snap the neck on it is there anywhere I could find this sort of part or will I have to replace my whole switch before I will be able to find the right external unloader valve. It seems to be a common looking part

    • Mike, you were going to send a photo of the unloader valve? We sent you and email with the address, but didn’t yet get the photo. Depending on the type of pressure switch and unloader though, I suspect you will have to buy a new switch to get a new unloader as spare parts for pressure switches are often few and far between. Watch the online used compressor listings. You may be able to pick up a used compressor – complete with pressure switch and unloader – for less than the cost of a new pressure switch, and then, you’ll have other spare parts too.

  22. When I start up my compressor the pressure gradually builds then around the 7bar level the safety valve blows out and the machine just carries on charging up until the safety blows out again and so the tank never reaches the red line so to speak when it should cut out properly rather it just chugs along cutting out below the 8 bar where I suppose the pressure switch should do the real cutting out process.

    Is this just a rubbish safety valve or is there something more at work here ?

    • If the PRV lets go before the compressor tank pressure reaches the normal cut out pressure setting, then we would surmise that it is the pressure relief valve that’s at fault. Sometimes oil in the compressed air can lubricate the piston inside the PRV and it let go prematurely. Try removing the PRV, soaking it in solvent, and after it dries, reinstall it and test the compressor. If that resolves the issue and the compressor pumps up to cut out and shuts down before the PRV releases, you’re good to go. We suggest too that if that works, make sure to cycle the PRV a few times manually to ensure that it will release on it’s own if needed. If the cleaning doesn’t work, we suggest a new PRV is in order.

      • thanks willr

        I’ve already ordered a brand new PRV in anticipation of some such problem.

        But you’ve pretty much confirmed that’s where the problem lies.

        I’ll replace the PRV when the new one comes then and have another go.

        If there’s any more trouble I let you know.

      • Hi I have a Webster 2 stage compressor model 66 5 hp 80 gallon. It was working fine you years until it blew a gasket at the unloaded valve cover so I changed the gasket . Started after the repair now it’s leaking though the unloader valve hole . Why is leaking there now?? Never took the valve out when I replaced the cover gasket ?

        • I was not able to see a picture of this compressors, so don’t know for sure, but am assuming the compressor has a tank? If so, that tank will likely have a tank check valve where the line enters the tank. If air is blowing out of the unloader valve when the compressor is stopped, it is quite possible that the tank check valve is not seating. It may need to be removed (tank empty of air, of course) cleaned or replaced.


    • Well, the first thing we would do is, with an empty tank and the compressor off, pull – test / clean – or replace the tank check valve. It is likely that the failure of the tank check valve to shut when the compressor is off that allows air to continuously leak out of the unloader valve. Get that one resolved first. You may then need a new pressure switch, but get the tank check valve leak fixed first.


        • Glad we could be of help with your compressor problem, Matt.

          • Hello i have a axminster 3HP 50L it builds up to 8bar the releases air back to about 6bar i have brought a new pressure switch and it still dose it can any one help thanks Henry.

          • Henry, empty the tank, pull, clean and replace the tank check valve. Did this work?

  24. Am having an issue with what I think may be an unloader valve. I’ve attempted to add a an air dryer between the cylinder outlet and the tank. I’ve checked and all my connections and no leaks. the elbow valve (unloader?) coming directly out of the cylinder head does not close when the dryer is in place. Take the dryer out of the loop and all is fine. WHen the tank is empty with no dryer in the loop the valve stays open until the tank reaches about 10psi and then closes.

    Any help appreciated

    • OK Peter, we wouldn’t put the compressed air dryer where you say you’ve installed it. Reason? The air coming from the pump is typically hot, has a high level of moisture and quite possible free water in it. By putting the dryer between the pump (what you call the cylinder?) and the tank, you are ensuring that the dryer is seeing the most inhospitable level of moisture possible.

      You don’t indicate the compressor make or model, nor what type of dryer. Since you email address suggests that you are in the cabinet making business, it were me, I would have a desiccant dryer installed in line just before each application where I was using air.

      Let the air compressor tank be the first step in de-watering the compressed air. If the air gets a chance to cool in the compressor tank, it will drop out a great deal of the water entrained in the air stream from the pump. Use an auto drain to ensure that the tank is drained regularly and frequently to prevent a water buildup in the tank which will, eventually, get back into the air stream as the air leaves the tank if it’s not water free.

      It is clear from what you have written that whatever dryer you installed is causing the problem. Rather than reinvent the wheel, take the dryer out of that location. We think that would be best.

  25. I am having the exact same issue as Travis. In my case I have a Kobalt 30 gal upright unit. Replaced the control unit (came with new un-loader valve) and same issue. Compressor fills to 125psi, shut offs but un-loader valve does not close and control turns the unit back on.

    Looking for any advice.



    • I think the issue is that your compressor tank check valve is leaking. The unloader valve is supposed to be open when the compressor is stopped, to vent air over the piston, aiding in the restart. If the tank check valve is compromised, all the air in the tank will leak out.

  26. Hi,
    I am having problems with my craftmen 2hp air compessor. When the pressure switch is in the auto position and reaches pressure, the switch hits the unloader valve and drains the tank until the next cycle. It also hits the unloader valve in the off position and totally drains the tank. I tried changing the pressure switch which included a new unloader valve and check valve but I still have the same problem. Do you have any ideas?


    • The unloader valve does not normally drain the tank, Travis. When the compressor reaches normal cut out pressure and the pressure switch toggles to off, to stop the motor, the unloader valves job is to open, and to drain any air trapped over the piston(s), helping the restart process when the compressor restarts.

      If your unloader valve is emptying the tank when the air compressor is off, the reason, typically, is that your air compressor tank check valve is leaking.

      Please read the pages about these items on this site to be sure that you thoroughly understand how they work together.

      • Ok I am also having the same problem as Travis. Been reading this site to help me troubleshoot. I got a new Condor pressure switch and replaced the old one, since it did not function at all, and had the same problem as travis. Gets up to 150PSi… drains down to 110 and motor kicks in again, repeats once up to 150. However, if compressor is unplugged around 130/140PSI no draining occurs. So, like this page suggests, I got the Craftsman replacement part specified on their website for my tank check valve, replaced it… issue persists, no change. It’s a 15 gallon horizontal craftsman compressor, oilless. I’m completely out of ideas. Help?

        • Hi Justin. Your compressor cuts out at 150 PSI, right? We assume this is the normal cut out pressure. At this point your pressure switch trips to cut the power to the motor. At the same time the pressure switch trips to off it opens the unloader valve, thus ensuring that the air of the piston is relieved.

          What keeps the air in the tank – since without it all of the air in the tank would leak out of the now open unloader valve – is the tank check valve.

          You say that your compressor “Gets up to 150PSi… drains down to 110 and motor kicks in again, repeats once up to 150”. How can the compressor tank drain down if the only source of the air is the air in the tank, and a functioning check valve keeps the air in the tank and stops it from bleeding out the now open unloader valve.

          Double check your check valve. It has to be leaking.

Speak Your Mind


Fix My Compressor Privacy Policy   |