How To Test Your Compressor Tank Check Valve

Ever wonder how to test your compressor tank check valve? Or even, why bother testing your compressor tank check valve?

What are the symptoms of a faulty compressor tank check valve?

When your air compressor stops after filling the tank, the compressor line from the pump to the tank becomes open to atmosphere. This is to ease the load on the pump the next time the compressor stops.

It is the compressor tank check valve that is in place to trap the air in the compressor tank, so that it does not bleed out of the tank back up the line from the pump, over to the unloader valve, and out to atmosphere.

In the image below we have a couple of typical air compressor check valves.

How to test your compressor tank check valve - www.fix-my-compressor.comThe compressor tank check valve is commonly threaded into the compressor air tank. The threads that are coated in reddish pipe compound are turned into a mating port on the compressor tank itself and tightened snugly.

Port 1 is where the line from the pump head would be attached to allow the compressed air to flow from the pump into the compressor tank.

Port 2 is where the air line that is plumbed over to the unloader valve is attached. These are compressors with an external pressure switch which may have an unloader valve on the side of it, or an internal unloader. While air is being pumped into the compressor tank through the port 1, the unloader valve is shut, so the compressed air cannot flow along the air line attached to port 2 and escape to atmosphere.

Port 3 is where the air from the compressor pump escapes from the tank check valve and into the compressor tank. It is within this part of the tank check valve that the actual check valve or flapper valve is housed.

When the compressor tank check valve leaks

If the check valve or flapper valve in the bottom of the tank check valve assembly becomes fouled up or breaks, air can escape from the tank. A typical indication of this is when the compressor stops, after unloading the pump, the unloader valve continuously leaks air.

That leaking air has to come from somewhere, and that somewhere is the compressor tank.

If that happens you need to test the tank check valve.

How To Test Your Compressor Tank Check Valve


test your tank check valve -

… blow into it. Yup, that’s how we test them.

Wipe the shank of the tank check valve up to the tank threads, wrap your lips around the valve below the tank threads, and blow.

If the tank ball check or flapper valve is good, no air should exit either ports 1 or 2. None.

If air leaks out of either of these two ports, your check valve is not seating, and that’s the source of the leak out of the unloader valve.

Try washing the valve in hot, soapy water, rinse thoroughly, and dry completely. Test again. If it still leaks, replace it.


  1. Matthew Brown says:

    If the tank check valve is faulty, can I just replace the internal parts, or do I have to replace the valve housing also?
    I. e. The entire brass unit

    • I suspect even finding parts would be prohibitive, and probably more expensive than a whole new unit. Never tried to find parts for a tank check valve myself. Just replaced the unit as needed.

  2. Paul Buchhorn says:

    Hi, great site, thanks for putting in the time to get it together. I have a problem with my compressor – from empty, the pump runs and fills the tank as per normal. At the cut out pressure, air comes out of the unloader as expected but then continues to come out. If I turn the power off, the air continues to escape until the cut in pressure is reached and then stops. If the power were on, the cycle would continue. Thinking it was the unloader valve, I replaced it but the problem continued. I am thinking it is a faulty pressure switch as opposed to a faulty one way valve based on the air flow from the unloader valve stopping at the cut in point. If it were the one way valve, I would expect air to continue to escape until the tank were empty. Does this sound correct. Thanks.

    • Paul Buchhorn says:

      Hi, as a test, I ran the compressor up and once it cut out, unplugged the hose that runs from the one way value to the unloader/pressure switch – at the unloader valve end, expecting only a short burst of air from the cylinder and no more as the one way valve would prevent this. But it continued to vent air till I pushed it back in to the unloader valve. So, either I am not understanding how the one way valve works in combination with the line to the unloader valve or the one way valve is faulty. If it is a faulty one way valve, then I am confused as to why air stops coming out of the unloader valve port once the cut in point is reached (when the compressor is off per my original post) as I would have expected the air to continue to vent until the tank were near empty. Thanks

      • Paul, the symptoms point to a failure of the tank check valve to seal properly. The air stops coming out of the unloader valve once cut in is reached as the unloader valve is closed by the pressure switch. The unloader is only open when the compressor is stopped.

  3. Bob French says:

    Hi-I have a Maxxus air compressor that suddenly started leaking at the “T” fitting shown in the attached pic. When I turn the compressor on, there’s a constant flow of air out of the cap which has appears to be a needle valve-
    Is this the compressor tank check valve?


    • willyr says:

      What that tee fittings looks like to us is one with a compression fitting on both sides, not a needle valve. But that may be just that we cannot see the image clearly. Try this. Shut the compressor down, drain all the air from the tank, pull the tank check valve with is usually where the line from the pump reaches the tank, and make sure the tank check valve is actually sealing up when there is air in the tank.

Speak Your Mind


Fix My Compressor Privacy Policy   |