Compressor is always leaking air

You know the compressor is always leaking air as when you shut it down with a full charge of air in the tank, after a short while you can see that the tank pressure gauge display is showing that the tank pressure is dropping. Soon, if the air compressor is plugged in, the pressure switch will react to the falling tank pressure, and start the compressor.

That’s a waste of energy and money, for sure.

The first thing to do is to determine where the leak on the air compressor is. Here are some tips on that.

No obvious leaks

If you can’t hear the leak, or feel it with your fingers, you can soap the connections on the air compressor.

First pull the air hose from the coupler on the compressor.

Use a tablespoon of liquid dish soap to a cup of luke-warm water. Brush this solution onto all the connections on the compressor, including into the end of the coupler.

Coupler opening -

Make sure you check up in here with the soapy water. Couplers have been known to leak.

If there is an air leak, bubbles will form where compressed air is escaping.

If you find a leak this way, tighten the fitting, and test again. Make sure there are no more leaks on the connections on the air compressor.

Check external unloader valve for leaks

If your air compressor has a pressure switch with an external unloader valve, make sure you soap it thoroughly as well.

Compressor pressure switch unloader valve -

If soaping the pressure switch external unloader valve – one of which is seen in this image – make sure your compressor is unplugged with lots of air in the tank. With the compressor off, the unloader valve should NOT be leaking any air at all after the unloading process, a second or two, has completed.

If you find that your compressor pressure switch unloader valve is leaking air continuously, then you have located (at least one of) the leak on your air compressor.

A pressure switch unloader valve that leaks continuously after the compressor has stopped indicates that the tank check valve has not closed completely, and compressed air is no longer trapped in the tank.

You will need to dump all the air in the tank, and with the compressor unplugged, disconnect the air line from the pump, remove the tank check valve, inspect it, clean it, or replace it, depending on its condition.

More information on tank check valves is available on this site. See the link Tank Check Valve under the Compressor Information list in the right column.

Does compressor only leak air with a hose plugged in?

If the compressor is only dropping when there is an air line plugged into the coupler, then the leak will be found downstream from the compressor coupler.

To confirm this, unplug the air line. If the air leak stops, the hose has a hole, or a connection downstream of the compressor coupler is the problem.

Still have a compressor air leak?

If you still cannot find the compressed air leak, add a comment below and we’ll see if we can help.


  1. Gregg Rhode says:

    I have a Devilbiss 6.5hp 80 gallon compressor that leaks at the manifold. My questions is what would I use to seal the manifold when I remove and replace.

    • Are you speaking of what kind of air fitting thread sealant to use? If so, I prefer to use pipe dope versus the plumbers tape as a sealant. Regardless of what you use, as long as it is applied with care, any should suit.

  2. Hello
    I just bought a Porter Cable pxcm201 and when it shuts off at 135lbs it leaks 5lbs of air within 30 – 40 sec and stops. It stays at 130 overnight. Do all compressors leak a few lbs? Should this be staying at 135 overnight? I soaped all the connection and can’t find a leak. It’s brand new and I want to find out where it is so I know if I should fix it or return it.

    • All compressors with an unloader valve will leak when the compressor reaches cut out and stops. That leaking air is voiding pressure over the pump piston to allow an easier restart. Typically that venting takes place in a second or two at most. Odd that yours vents for 30-40 seconds, though since it only loses 5 PSI over that quite a long amount of time, that venting is fairly slow. I’m guessing first a sluggish tank check valve, and if not that, then a sluggish unloader valve could be the culprit.

      • Thank you. I should have been more clear about it. Yes it releases air when it reaches cut out pressure and all is quiet, then if looking at the gauge it slowly drops 5lbs. Would you return it or fix it?

        • John, difficult to be sure. Without pinpointing the leak. If you can take it back for a replacement, I suspect that’s what I would do.

      • Also, I just did another soap test. I started the compressor and put soap bubbles all around and over the hole of the unloader valve immediately after it unloaded, watched it like a hawk and the bubbles didn’t budge. I don’t understand why I cant find any leaks anywhere unless the soap test isn’t fool proof. Sorry for the replies but it’s brand new and driving me nuts

        Thank you

        • That it stops after 5 PSI makes me wonder if it’s the gauge itself that’s leaking?

          • Wow I didn’t think of that. This weekend I plan on checking/cleaning the tank valve and if it’s ok I’ll move on to the gauges. Thanks for all your help

  3. I have a slow pressure leak in my 30 gallon, 150 PSI vertical compressor, but it stops leaking when it drops to the 100 to 95 psi range, then can hold and not leak any more, with nothing connected. I’ve not yet timed how long it takes to get down to that range. This is with no hoses or tools connected. I’ve soaped all the joints but see no bubbles. Any ideas?

    • I suspect that you’ve got a pressure related leak in the tank check valve, which stops leaking once the pressure drops below that threshold. Empty the tank, remove, clean, test and replace the tank check valve. Did that make any difference?

  4. Model-man-Chris says:

    My iwata moisture trap leaks whenever I try to get about twenty psi. When I turn it up it will stop leaking air from the bottom stops leaking. The compressor is suppose to turn off automatically but doesn’t seeing how the moisture trap is leaking. Help please I am new to airbrushing.

    • You are referring to the combination Filter / Regulator installed on the discharge line to the discharge coupler on the Iwata compressor? Have you, with the compressor off and the tank, if any, drained of air, removed the bowl from the filter and emptied it?

  5. Brad Smith says:

    My compressor holds air just fine but when i turn the knob to direct air to the attached tool there is a huge air leak out of the rear of the manifold. What do i need to do to fix. Replace manifold internals?

    • Brad, by the “the knob to direct air” are you referring to the regulator knob? Does the leak continue after you turn the knob or does it stop. What make and model of compressor, please?

      • Brad Smith says:

        Kobalt Air Compressor F15DKL, F215DKL, or 232177 according to “” when i turn the regulator knob to send air to the air hose, a huge air leak is coming from the rear of the regulator housing. There is a flat plate with four screws that it leaks out of. I have dissasymbled, cleaned as well as possible but still leaks. It holds air fine until you try to direct air to the tool. Also was oily inside. Is that normal?

        • Brad, I couldn’t find much info on this F15DKL Kobalt, so I’m not sure that it’s oil lubed. From the image I would guess not. So, where did the oil inside the regulator come from? If not from the compressor then it must have been there when assembled. I did see complaints about the regulators failing on this model. I suspect the diaphragm has let go. Since I could not see if this regulator is in a manifold, or just attached to one, I cannot tell you how easy it will be to replace, yet since you can buy a new mini-regulator for $15 or so, it might be worthwhile doing that rather than spend any more time trying to fix an inexpensive, no-parts-available regulator.

  6. Hi,

    Bit of a newbie to this but, should a moisture trap always leak air to the extent that the pressure is always falling when the compressor turns off rather than waiting for a demand. Mine is leaking 5 Bar in 10/12 mins.

    • Nope, Matt. The compressed air filters (moisture trap) should only expel air when the manual valve at the base of the bowl is opened, or, if it is a float drain and there is sufficient water in the trap to lift the valve to open the drain.

  7. I have Rigid, 5 gal compressor. When I turn it off and unplug the power cord, the air releases out of the house or connector until there is no air or pressure left in the tank. I don’t know if this is a pressure switch issue or something else.

    • Zac, I’m not quite sure what you mean by the “air releases out of the house or connector”. If that’s supposed to say hose, are you saying that when you plug the connector into the coupler, air is leaking out there? If so, you may have a mismatched coupler and connector. If the air is not leaking out there please advise where.

  8. Hello. First, thank you for such an informative helpful site. Now the problem. I have an older 60 gallon Ingersoll Rand Compressor model IR5E6VA. Works great with the exception of the air that continually leaks through the oil filler/breather plug. From what I’ve learned here I’m suspecting the cylinder seals. I think that’s right? It leaks wether it’s running or not thereby depleting the pressure in the tank causing the compressor to cycle on and off. No oil blow by. What I’d like to hear is “easy fix”. Thanks again.

    • The compressor symptom strongly suggests that the piston seals are badly worn or damaged. A little air will wick past between the piston and the cylinder walls all the time, yet as the seals wear or are damaged, more and more air gets through, and that air blows out the oil fill vent. That air is leaking after the compressor stops seems to us to be a problem with a leaking tank check valve, too. Easy fix? Depends on your skill level in tearing down the pump. The tank check valve is fairly easy, and that’s covered on a page on this site already.

  9. 60 gallon vertical tank. Plug in side is leaking. Removed and replaced the “O” ring and wrapped threads with teflon tape. Screwed plug in and tightened with 24″ pipe wrench. Still leaking. Any suggestions? Thanks,

    • Odd that, George. We’re not fans of teflon tape in air systems anyway. We suggest you try using a paste thread sealant, which offers more mass to help seal up pinhole leaks.

  10. Denver James says:

    Hello, I have a small dual tank compressor that sounds like air is coming out within the motor housing. Any ideas what that could be?

    • Make… model? It’s hard to be specific without specific info. Offhand, we’d suggest you check the tank check valve to be sure it’s working.

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