The air compressor unloader valve

The air compressor unloader valve provides a critical function in preparing your air compressor to restart.

A failed air compressor unloader valve is one of the most common reasons why your air compressor cannot restart after you have used air from the compressor tank. When the the air compressor unloader valve is working correctly, the pressure in the tank has dropped to below the normal cut in pressure level of the pressure switch, the pressure switch trips to allow power to flow through it, and the motor starts. If the unloader valve has failed when the motor tries to start quite often the motor will lug (struggle) to start and typically a breaker will trip or a fuse will blow.

What the air compressor unloader valve does

Compressor unloader valves are common pieces of equipment on many types of air compressors including reciprocating, rotary screw and even gasoline or other fossil fueled air compressors.

This Fix My Compressor website is focused on the do it yourself and small workshop air compressor and most of these are reciprocating type air compressors. A reciprocating air compressor will have at least one cylinder, and often may have two or sometimes even three cylinders. Reciprocating air compressors always have an unloader valve.

In the photo below the unloader valve connections are the brass fittings on the left sides of these Lefoo brand pressure switches. The pressure switch on the right incorporates an ON/OFF lever on the right side as well.

Lefoo air compressor pressure switches

When the reciprocating air compressor reaches the cut out pressure setting the power supply to the compressor motor stops, and as a result, the compressor pump stops. The compressor pump stops, regardless of where the piston in the cylinder is located, and that often means that there is compressed air trapped over the piston when the pump stops on cut out.

On other pages on this site we talk about how marginal a 120 volt air compressor motor really is, and the steps the motor manufacturers must take to ensure that, even with all compressor components working at their best, even being able to start.

If air is trapped over the piston on the air compressor, that adds load to the start circuit. Your 120 volt power supply hasn’t got enough oomph to start the motor without help from a start capacitor anyway, and that additional load on the compressor motor is typically sufficient to prevent the air compressor from starting.

The air compressor unloader valve vents trapped air

What the unloader valve does is that is responds to the compressor pressure switch turning the power off to the motor and allows any air that may be trapped over the piston, or pistons on a multi-cylinder air compressor, to vent to atmosphere, removing any load that the trapped air may have created over the cylinder piston.

How the unloader valve works

The pressure switch may have an external unloader valve like the Lefoo pressure switches above, or it might have an internal unloader valve the the Condor type switch shown below, which incorporates an ON/OFF switch on top though not all of them do.

Condor air compressor pressure switch

What you cannot see is the air fitting (could be quick connect or another style) underneath the Condor into which the air line from the compressor pump is installed.

Regardless of how the air line from the pump head is connected to the unloader valve in your pressure switch ~ an internal, underneath connection like the Condor or an external connection like the Lefoo (there are more brands, I’ve just used these two popular ones to illustrate the concept)  ~ the unloader valves do the same thing.

When the air pressure in the tank reaches the cut out pressure of that pressure switch, the switch trips to off, cutting power to the compressor motor.

Unloader valve opens

The process of turning the power off to the compressor motor is an electro-mechanical one. That means that electricity is driving it, but the end result is a mechanical movement. Something moves inside the pressure switch housing.

What is moving, typically, are a set of points which open and close depending on the pressure in the tank.

The movement of those points is connected to an arm or finger inside or outside of the pressure switch, and that arm or finger pushes or pulls on a small valve pin, which operates the air compressor unloader valve.

When the pin on the unloader valve is depressed, the air path through it is open to atmosphere, and any air trapped over the piston vents out. This happens when the compressor stops or cuts out.

Unloader valve closes

When the pressure in the compressor tank drops to the cut in pressure setting on your pressure switch, the pressure switch will trip again, and this time the arm or finger will typically release the pin on the unloader valve allowing it to shut, and prevent the air that is being compressed from escaping to atmosphere.

There are two unloader valves shown in the photo below. On the left is the Condor version. This unloader would typically be inside the pressure switch with just the brass insert fitting that is on the bottom of this unloader valve showing. The internal arm or finger to operate this unloader is not shown, but the pin which is depressed and released by that internal arm or finger is shown on top.

Condor-Lefoo unloader valves

The image on the right just above is the external unloader valve. You can just see the small pin protruding from the bottom of the unloader valve, and that little grey blob just below the pin is the arm or finger extending out from the pressure switch and is what depresses or releases the pin as the pressure switch trips on and off.

What about the air in the compressor tank?

I expect you have a question at this point? When the unloader valve is opened to atmosphere, why doesn’t all the air in the compressor tank escape too? For the answer to that, you will have to see the tank check valve page.

Your comments or questions are welcome using the form below.


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  1. john buxton says:

    I have a Tahoe gasoline 3 cylinder compressor that will run at idle,how should I adjust the unloader valve to start the compression phase? It is a new unloader valve because I let my brother use it and I suspect that he adjusted the unloader valve and it blew a head gasket on one cylinder of the compressor,which I repaired and it was working as it should until it began to only idle.

    • We don’t work on gasoline or diesel powered air compressors, sorry. Happy to post this for you in case a kind visitor can comment.

  2. PanamaJeepster says:

    Hoy there! Thanks for this site. I’m installing a small air compressor under the hood of my 4×4 and plumbing it to an air tank. I already have check valves but do I also need an unloader and if so what are my options? It’s a 2 piston with 1/8″ fittings and I’m using 1/4″ ID rubber air hose.

    • I guess that question is, if the air compressor comes without an unloader valve, why install one? It has a pressure switch to control the ON/OFF, yes? If so, and if the compressor cycles ON and OFF OK, then we suspect that it’s a small, fractional HP compressor motor, and an unloader isn’t necessary… or, under the hood is a small hole in the plumbing from the pump to the tank, and that hole will act as an unloader once the compressor stops.

  3. I have a small Porter Cable twin tank that needed a new unloader valve. The new valve assembly has a quick connect as opposed to a threaded one on the old valve. The unloader tube is threaded though and there does not seem to be a replacement of any sort available that I can find. Is there an adapter for this situation?

    • We cannot see the image, but here’s what we would do. Visit your local air valve / air cylinder distributor. Google it. Buy from them a couple of inches of tube that is the same OD as the unloader tube. Also get one straight instant fitting, same size tube, with the same male or female thread as the existing unloader tube. Push the new tube into the instant fitting on the unloader, thread the tube for the old unloader onto the fitting, and then push the tube into the instant fitting, and you’ve made a patch. Did it work for you?

  4. Bryan Marsden says:

    I have a dewalt 2 gal. Oil less compressor. It turns on and builds pressure to 150 psi. Then you can hear a small leak coming from the bottom of cylinder piston ring. It also will not recycle on when in use, the motor will just surge but it will not start unless you manually release the pressure to 30 psi.

    • Bryan, is this the one with the small pin hole near the piston, or does it have a pressure switch with an unloader valve. If it has the pin hole, that pin hole acts as the unloader, venting air over the piston when the compressor stops. If it continues to leak out that hole, then you’ve got a tank check valve issue. As to not restarting, if the unloader (pin hole or valve) is working, then your next step, assuming you are not using an extension cord, is to check the start cap on the motor. See how on this site.

  5. I have a craftsman 150psi 33gal compressor, it will run until full then when it shuts off, air escapes behind the pressure switch in large volumes until the switch tries to turn the motor on again. It won’t start and hums until the circuit breaker triggers. The off / auto switch seems to not work and I cannot get it to restart. I have to unplug the compressor to avoid the breaker from triggering sparking when I remove the plug from the socket. Leaving it sit for awhile it will start again then the process continues. I have replaced the pressure switch and the unloader valve to no avail?

    • Dan, see the page on air tank check valves, and check to see if yours is leaking. We think it is. If so, clean it up or replace it, and see what symptoms persist. If some do, add a comment here please.

  6. Chuck Bouchard says:

    Hello Willyr,
    I just bought a Porter Rand 4 h.p. Compressor model CPF 23400S. It works fine but kicks out a 15 amp breaker on every restart. Is it possible for me to add on a decompression valve to this compressor and where and which parts would I buy? Thanks in advance.

    • Chuck, we’ve never heard of a Porter Rand compressor. Porter Cable has one with this model number, however. As to the decompression valve, if you look at the pressure switch on this model you should see that there is an unloader (decompression) valve on the side of it. You might test it according to the instructions provided on the troubleshooting pages on this site.

  7. Just bought a Harbor Freight 21 gallon air compressor….I have it plugged into a switched outlet 30 amp. It runs fine using the through switch on the compressor but not if I use the wall switch. It seems that this model has the unloader valve built into the through switch on the pressure switch. Is there a way to install a automatic unloader so I can use my wall switch……?

    Thank You Brian

    • The pressure switch cycles the compressor ON and OFF as the tank pressure changes. In order for the pressure switch to turn the compressor motor on, power must be flowing to the switch from the wall plug. If you have a switch in the circuit that cuts the power to the power plug for the compressor, even if the compressor needs to start, it can’t, as no power is flowing to the pressure switch from the supply circuit. If you turn the wall switch on, and power is now supplied to the compressor pressure switch, and that switch is tripped to passing, the compressor motor should start. If you have cut the power to the pressure switch mid-fill, then the compressor will stop. Now it has air over the piston, which will overload the start and perhaps not allow the compressor to go. Either let the compressor run to fill and shut down before cutting off the wall switch, or install an 2/2 120 volt air valve in the unloader line, powered through the supply wire from the wall switch, so that when you turn off the switch the valve opens and dumps the air over the piston.

  8. I have small air compressor with electric motor, the problem is tube that connect check valve to unloader valve is broken and i can’t find similiar tube around my place. can i change that tube with ordinary pneumatic tube?

  9. Michael J. Berbari says:

    Why does the unloader port need a line to take it to the switch/shut-off??? All it does is let a little relief out, why not just have it release at the check valve area. Why the need for the line going to switch box, it doesn’t operate anything.
    Thank You

    • We thought that was covered on the air compressor unloader valve Michael (which is where we have moved your post to). I guess we weren’t clear?

      When a compressor stops, air is often trapped over the piston. If that air remains, when the compressor tries to start, the motor may not be able to overcome the additional load of the trapped air, and a fuse may blow or a thermal cut out might shut down the motor. The trapped air has to be vented.

      Unless you have a really small air compressor that vents air all the time the compressor is running through a small hole in the line to the tank, the venting of air over the piston takes place when the compressor stops. The venting, or unloading, is accomplished by opening a small air valve – appropriately known as the unloader valve – which allows a path of air from over the piston to atmosphere. The unloader actually opens a path for the air in the tank too, however the tank check valve traps the compressed air in the tank, and keeps it there, even though the unloader valve is open to atmosphere.

      In other words, the compressor must vent air from over the piston when the compressor is stopped, but not vent air (unless it’s a really small compressor) to atmosphere while the compressor is running.

      Hope this makes sense.

      To do this, there must be a valve involved, and rather than adding another valve at the tank check valve area, and installing a mechanism for turning it on and off, many air compressors incorporate the movement created within the compressor pressure switch as the points open and close, which has to cycle to turn the compressor on and off anyway, to also open and close the unloader valve.

      Of course, the air line from the pump head also has to be plumbed over to the unloader valve which is, typically, on the side of or inside the pressure switch.

      And that’s why that line exists.

  10. The air compressor unloader valve first start unloading after ca. 20 seconds after cut off it’s a condor mdr2 aev2s unloader valve can you help

    • When the air compressor is off, the unloader valve should be open all the time. It opens at cut off to dump any trapped air over the compressor piston, and then it stays open until the pressure switch trips to ON to allow power to flow to the compressor motor and start the compressor. If the unloader valve is leaking air all the time the compressor is off, then it is likely that your tank check valve is not sealing properly.

      • the unloader valve opens first after 20 seconds so the engine can start it’s been a long wait, it’s a whole new condor MDR 2s with AEV 2s unloader valve sincerely Ole

  11. Hi guys, each time my silent oil-less compressor fills up the tank and cut off, the unloader valve makes an annoying loud burst of air. I wonder is there such a thing silent (quieter) unloader valve that I can replace it? Or is there a way to fix a muffler/silencer to the valve? Its an internal unloader seated inside the pressure switch behind the 2 electrical wire ports.

    • We don’t know of any pressure switch with integral unloader that has a muffler, Wong. Given how the pressure switches with internal unloaders are plumbed, and even the ones with the unloader valve on the outside for that matter, attaching some sort of muffler is problematical. If the noise is really ornery, use a longer hose from the compressor, and put it under the work bench or in the corner, or some such?

      • Hi willyr, thanks for the reply. You were spot on how the bleeder valve seated deeply in the pressure switch unit, is impossible to fix any silencer/muffler. Sadly due to space constraint in my airbrushing room, there is no where I can relocate this 0.75hp w/ 8Gal tank compressor, sigh gotta live with the loud short burst.

  12. Hello,

    I was wondering….can a plastic unloader valve be replaced with a brass one? Can an unloader valve be replaced by any other type? I have a Kobalt TQ3126 and the unloader valve failed. It looks like it shorted (black residue under the pressure switch). The tubing that connects to the compressor inlet piping split open.


    • Hi Isaac. To answer your questions in order… yes, and yes. As long as the pump unloads when the compressor reaches cut out, it doesn’t care what type of unloader does the job. If you prefer an internal unloader valve, change pressure switches to one that has the internal unloader. If you prefer one with an external unloader, any brand will do as long as you wire the pressure switch correctly, it has the same cut in and cut out and voltage of the original, is plumbed similarly, and you can connect the unloader line to the unloader valve.

      Of course, if your compressor is under warranty, you’ll want to use OEM parts to maintain the warranty. Otherwise, use whatever parts work.

  13. I have a Campbell Hausfeld compressor model VT623301AJ with a VT470200KB pump that hadn’t been used in a few years. I fired it up and it has been having difficulty running and will trip the breaker.

    This is what I have so far. When the line is open so no pressure will build it starts up immediately and smoothly and runs no problem. If I shut off the line to build pressure you can slowly hear the difference in the pressure load on the pump and eventually it will trip the breaker at ~90 PSI instead of the 125 PSI cutoff. If it has low pressure ~30 PSI in the tank the pump will try to start but before it reaches top rpm it shudders and trips the breaker. At the higher tank pressures 60+ PSI it tries to move under load from what it sounds like and trips the breaker. Now with the tank under pressure and switch off the unloader valve does not leak but when the switch cuts off or is turned off there is a hiss that follows that sounds like the unloader relieving pressure. So is this the unloader or the check valve or some combination of both? Could it even be the start/running capacitor on the electric motor?

    • Well, we think you’ve determined that it’s not he tank check valve leaking, or a failed unloader, as from your post it sounds like the unloader is working find. We think you may have an electrical issue. First thing to check is to make sure that you’ve got clean power, preferably on a 20 amp circuit, so that the power supply to the compressor is not compromised. If you have any extension cord or power bars into which the compressor is plugged, get them out of the circuit. Plug the compressor directly into the wall socket. If you’ve done this, or this doesn’t help, then we would be looking at the capacitor as being weak or failing. Check that next, and then it may require a motor load check too, but first, we would check the caps. See how that’s done on the page on this site.

  14. This type of unload mechanism won’t work with head unloaders will it? the unloaders on my pump were operated by positive pressure from a pilot unloader valve. Im converting a gas powered job to electric and the whole unloader conversion has me more than a bit confused

    • Dale.You are correct. The unloaders shown are for compressors powered by an electric motor. Once you get into alternative fuels, the unloader valve does different things. In your compressors case, your unloader adjusts the throttle valve, so that, when the compressor reaches cut out, the motor is throttled back to idle until the demand for more air, and then the motor throttles back up again.

      When converting the compressor from gasoline to electric, the unloader valve will then vent air over the piston(s) when the compressor reaches cut out pressure and STOPS, this allowing an easier re-start to the compressor motor when the tank pressure drops and the motor has to start to pump more air.

  15. rffs1ham says:

    I have replaced my pressure switch with a Pressuremate 034 -0197RP. When the pressure reaches the 130 psi lever the motor is shut off and air starts coming from the pressure switch. When the pressure leaks down to 90 psi the motor is switched on and the air leak stops. This is the second switch I have installed. What’s happening?

    • We can feel your frustration from here! What we think you’ve got is a bum tank check valve. Reason is, when the pressure switch trips off, stopping power to the compressor motor, the unloader valve is tripped open at the same time, to allow any air over the piston to escape. If you have a leaky or failed tank check valve, along with the air that comes off the piston head, all the air from the tank will leak out too, at least until the pressure drops to cut in, and the compressor starts. When the compressor starts, the unloader valve is tripped off, closing it. That’s what we think anyway, based on your comment.

      • rffs1ham says:

        Is the unloader valve in the pressure switch?

        • It depends on the make and model of the air compressor. If your air compressor has an external pressure switch, then typically the unloader valve with either be found attached to the side of the pressure switch. If the unloader valve on your air compressor model is integral with the switch, the unloader will be inside the switch cover, and the unloader line typically connects to the bottom of the pressure switch via a quick-connect type of fitting.

          Some small air compressors have a tube-like pressure switch inserted into the compressor tank. That type of switch does not have an unloader valve associated with it.

          • rffs1ham says:

            Is there a check valve in the 1/2 in. line that feeds the tank? I have an external pressure switch that had the unloader outside of the pressure switch. i’m confused with the what the tank check valve is. Is it’s function to keep air from the 1/2 in. feed line to the tank and flowing through the compressor and through the 1/4 in. line that goes to the unloader valve?

          • Please review the page Compressor Tank Check Valve linked from the right navigation bar which explains what it is and where it is on a typical air compressor.

  16. Danny V. says:

    Hello, i have a sears 31 gal air compressor that starts great, but after it cuts out it is under strain to restart and trips the circuit breaker. It does not hiss when it cuts outs. Any ideas on why this happens? When motor is cool and tank is empty it starts right away. Could this be a unloader or switch problem? Thanks.

  17. Hello. I have an air compressor where the relief valve starts popping open and closed when the compressor shuts off. Would anyone have an idea as to what I can do to repair that? The tag on the valve says, ” A popping relief valve is an indication of excessive interstage pressure.” I’m not sure what that means,or how to fix it. Thanks in advance for your help with this matter.

    • willyr says:

      A bit hard to be specific as we have no idea what make or model of air compressor yours is. If the PRV you refer to is in the line between two cylinders, then you quite likely have a problem with the intake valve on the second cylinder.

  18. aloha, , i have a Campbell hausfled 5 hp, 30 gal, (vt629901aj) compressor it has started to develop this straining sound when trying to build up pressure above 80 psi. i have cleaned and checked the valve in the tank, it builds pressure up to 80 psi at normal speed ,then the motor seems to slow to about half speed then it strains further and will stop. at this point i unplug it, , do i need a pressure switch or the unloader vavle, ?any help would be greatly appreciated, thank you john

    • And Aloha back to you. While there could be many things that may be the cause of the motor straining as back pressure from the tank grows, one of the most common is a failing run / start capacitor. Please see the pages on this site about what they are and how to check them. After doing so, if they are not the cause, please add a comment and we’ll on to the next logical source of the problem.

  19. I don’t really how to describe it. The compressor act like it’s in a bind. It will run pretty good then slow down as if in a bind, The compressor is full oil. The brand is a Kobalt from Lowes. thanks in advance

    • Does it run up to cut out pressure and then stop, or is it “binding” while the compressor is pumping up to cut out? Are you using an extension cord? (If so, don’t unless it’s a short, heavy duty one.) The “binding” may be a power supply issue, but it could also relate to back pressure buildup overloading the motor.

  20. I’ve taken my 3 gal compressor and removed the compressor from the tank. I them put an old refrigerator compressor on in its place, but now I’ve got air leaking from the unloader. It won’t allow the air into the cylinder to compress. Is it because of the low air flow? a bad check valve or the unloader?

    • I would think that if you have air leaking from the unloader valve that it is the tank check valve that is the problem. When the air compressor is off and the unloader valve is open then it is the tank check valve is what keeps the compressed air in the tank.

  21. Hi,
    I have a 2cylindr oil free Gentilin air compressor C330/20 which is malfunctioning due to air leaking from unloader valve. This unloader valve fixed with pressure switch.

    During pumping air leaking and compressor running continually and trying to fill up the tanks until the cut out point (10bar) has reached. But it won’t reached at cut out point because of air leaking, so it continually pumping and as the result air supplying tube to unloader valve ruptured. Because of continuous pumping air lines and piston heads are getting hot enough to burn if touched.

    Any help will be appreciated.


    • Imran, though I am not familiar with the compressor, I did find the catalog and had a look, as best I could, of the setup. Based on what you are saying, it seems clear that the unloader valve in the pressure switch is stuck open, and, the result is exactly the symptoms you describe. If this were my air compressor, I would be replacing the pressure switch to resolve the problem. The easiest solution is to purchase the same switch from the distributor of the Gentilin compressor, though I suspect, as many compressor manufacturers do, they buy a standard switch from a switch manufacturer, in which case, you may be able to find a similar pressure switch with an integral unloader from any compressor or fluid power shop.

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