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Air Compressor Tank Check Valve Troubleshooting – Testing & Repair

Published Categorized as Air Compressor Troubleshooting, Air Compressor Valves No Comments on Air Compressor Tank Check Valve Troubleshooting – Testing & Repair

Compressor tank check valves fail from time to time. Here are some troubleshooting tips for buying a replacement compressor tank check valve when that becomes necessary.

Typically a compressor tank check valve failure occurs due to contamination building up inside the check valve, which prevents the flapper or seating ball from closing tightly enough to prevent air back flowing through the valve.

For more information on how air compressor check valves work, what they do, and why they exist – check out our guide explaining everything you need to know about Air Compressor Check Valves

Table of Contents

Symptoms of a Tank Check Valve Nearing Failure

Detecting symptoms of a tank check valve nearing failure early can help avoid significant complications and costly repairs.

Intermittent Air Leaks

Description: One of the first signs of a tank check valve showing wear is the presence of periodic leaks. These leaks are especially noticeable during the startup and shutdown phases of the compressor.

Reasons: A gradual deterioration of the check valve’s seal or gasket can lead to minor air breaches.

Preventative Measures: Conduct regular inspections of the seals and consider replacements if wear is detected. Opting for high-quality seals known for longevity can also prove beneficial.

Testing Method: Keep a close watch on the compressor during its startup and shutdown phases. Irregular air escapes can point towards impending check valve issues.

Delayed Startup

Description: If your compressor takes longer than usual to start or to achieve its desired pressure, it’s a clear sign that the check valve might be inefficient in maintaining pressure.

Reasons: Over time, deposits or wear might affect the check valve’s ability to hold pressure.

Preventative Measures: Clean the valve at regular intervals to avoid any clogs and always maintain the recommended pressure levels.

Testing Method: By timing the duration from startup to when the compressor reaches optimal pressure, you can determine any inefficiencies. Compare this with typical durations to identify potential valve problems.

Fluctuating Pressure Readings

Description: Inconsistent readings on the pressure gauge can be indicative of the check valve’s struggle to maintain a stable pressure in the tank.

Reasons: Wear and tear or potential obstructions in the valve can cause such fluctuations.

Preventative Measures: Regularly calibrate your pressure gauge and ensure that the valve mechanisms are free from any debris.

Impact on Other Components: A fluctuating pressure can strain the compressor pump as it constantly tries to stabilize the pressure.

Visual and Functional Anomalies

Description: Over time, you might notice your check valve getting “sticky” during operation or showing visible signs of wear. Early stages of corrosion might also become apparent upon closer inspection.

Comparative Analysis: A healthy check valve operates smoothly and shows no significant signs of wear or corrosion. Visual aids or diagrams can help distinguish a functioning valve from one nearing its end.

Preventative Measures: Ensure that the valve is cleaned and lubricated regularly. Avoid conditions that might accelerate corrosion, like excessive moisture.

Impact on Other Components: If not addressed, contaminants from a deteriorating check valve can spread to other parts of the compressor system, potentially causing more widespread issues.

To ensure the continued optimal performance of your air compressor, being proactive is key. Addressing early symptoms, especially those related to the tank check valve, ensures the longevity of the system. Regular maintenance, combined with a keen awareness of these symptoms, can save both time and money in the long run.

Symptoms of a Tank Check Valve That Has Completely Failed

Recognizing the symptoms of a completely failed valve is essential to maintain operational integrity. Here’s an in-depth look at these symptoms of a bad check valve.

Continuous Air Leaks

Description: Air is constantly leaking from the unloader valve, especially when the compressor is halted.

Reasons: A compromised seal in the check valve, leading to an inability to hold back the air.

Comparative Analysis: Unlike intermittent leaks that occur periodically, this is a non-stop leak, indicating a complete failure.

Test and Troubleshooting Methods: Listen for a hissing sound or use a soapy water solution around the valve to spot bubbles. If confirmed, a valve inspection or replacement is necessary.

Preventative Measures/Actions: Regular maintenance checks and timely replacements before complete wear-out can prevent such issues.

Impact on Other Components: Continuous leaks can reduce pressure, affecting the efficiency of air tools and stressing the compressor motor.

Compressor Never Reaches Cut-Off Pressure

Description: The compressor runs continuously without ever reaching the set cut-off pressure.

Reasons: Failed check valve not maintaining the pressure, causing the compressor to overwork.

Comparative Analysis: A healthy compressor will cycle on and off once it reaches its cut-off pressure. Constant operation without cut-off indicates a fault.

Test and Troubleshooting Methods: Monitor the pressure gauge. If the compressor doesn’t cut-off after a prolonged period, it’s a clear symptom.

Preventative Measures/Actions: Regularly inspect the check valve for wear and ensure it seats properly.

Impact on Other Components: Overworking can cause the compressor motor to overheat or fail prematurely.

Persistent Leaks During Operation

Description: Air constantly escapes through the unloader valve while the compressor is running.

Reasons: The check valve might be stuck open, allowing backflow towards the compressor.

Comparative Analysis: In a fully functional system, there should be no leaks during operation. Constant leaks suggest complete valve failure.

Test and Troubleshooting Methods: Check for leaks during operation. If air is escaping continuously, inspect and replace the valve.

Preventative Measures/Actions: Ensure that the check valve is free from debris and clean it periodically.

Impact on Other Components: This can lead to reduced operational efficiency and increased wear on the compressor.

Zero Pressure Build-Up

Description: The compressor can’t build any pressure.

Reasons: The check valve might be stuck open, preventing pressure buildup.

Comparative Analysis: Unlike fluctuating pressure, zero pressure indicates a more severe issue, potentially a completely open valve.

Test and Troubleshooting Methods: Attempt to run the compressor. If no pressure builds up, check the valve for obstructions or damage.

Preventative Measures/Actions: Regular inspections and avoiding overpressurizing can help.

Impact on Other Components: Without pressure, air tools won’t function, and the compressor becomes redundant.

Excessive Moisture Issues

Description: Presence of water in the compressed air.

Reasons: Failed valve due to corrosion from moisture, allowing condensed water to mix with the compressed air.

Comparative Analysis: A slight moisture accumulation is an early sign, but excessive moisture indicates a significant valve issue.

Test and Troubleshooting Methods: Use moisture indicators in the air line. If they turn color, moisture is present.

Preventative Measures/Actions: Installing air dryers or moisture separators can help.

Impact on Other Components: Moisture can damage air tools and cause rust in the air tank.

Visible Damage on the Valve

Description: Physical damage like severe corrosion, cracks, or deformities on the check valve.

Reasons: Wear and tear, moisture damage, or excessive pressure can lead to visible damage.

Comparative Analysis: Minor wear might not affect functionality immediately, but visible damage often indicates a non-working valve.

Test and Troubleshooting Methods: Regular visual inspection can help identify any deformities or damage.

Preventative Measures/Actions: Keeping the compressor environment dry and ensuring proper pressure levels can prevent such damage.

Impact on Other Components: Damaged valves can allow backflow, affecting the compressor’s internals.

Recognizing the symptoms of a completely failed tank check valve is vital. By understanding each symptom, testing and troubleshooting, and taking preventative measures, you can ensure the longevity and optimal performance of your compressor. If any of these symptoms are detected, prompt action is recommended. Operating with a faulty valve not only reduces efficiency but also poses potential safety risks.

General Tips for Check Valve Inspection

It’s critical to also conduct regular inspections on all parts of the air compressor including the check valves. This can include but not limited to:

  • Checking the valves for leaks
  • Looking for any signs of corrosion, rust or mineral build-up
  • Ensure you replace the valves if there are extensive leaks or damage
  • Open & close the valves to ensure they are not seizing up
  • Conduct an inspection on the pressure and temperature of fluids flowing through them
  • Ensure they’re not close to or over the rated limit for the valves

If you need to clean everything, the valve must be disassembled and taken out. Hydraulic fluid can then be used to wash this, or whatever fluid is present in your system.

How to Clean Air Compressor Check Valves

Cleaning the check valve first involves removing it from the compressor tank and disconnecting any air lines as that is accomplished.

Blowing air through the check valve in the opposite direction of normal flow, using some sort of solvent to clean out the inside of the check valve, and sometimes simply flushing them with hot water can resolve the issue.

It is important to keep your check valves clean so that damage is prevented. A towel can suffice to wipe excess dirt and dust off, if the debris is plastered all over the valve a wire brush can be of great use. It’s important to keep the valve lubricated as this can ensure optimum operation and better efficiency. Lubrication will keep you from having to replace or repair the check valve so soon.

If those fail to clean the valve then that means it’s time to replace the check valve and buy a new one.

Common Issues With Compressor Tank Check Valves

Leaky Tank Check Valve

Many folks, with an air leak at the pressure switch, may think that the switch itself is leaking or that the unloader valve has failed, and is allowing compressed air to bleed out of the air tank.

While a leaking unloader valve is a possibility, the more frequent occurrence is that the check valve has not seated properly, and is allowing air to bleed out of the tank, when normally it should be shut tight allowing no compressed air to escape.

Depending on how much debris was in the check valve seats or the level of check valve failure that had occurred, would determine whether the leak out of the unloader valve was slight or a lot of wasted compressed air.

If you have a leaking unloader valve, first make sure the compressor power is off, that the tank is drained of all compressed air, then remove and clean or replace the check valve assembly to stop the leak.

The check valve can also break down and on occasions cause a significant amount of oil to pour out the inlet.

Cold Start Valve Leaking

Having an issue with your cold start valve leaking on your check valve?

If the “cold start” valve is leaking, the air has to be coming from the tank. If the cold start valve is truly a valve, then its purpose is to allow air out briefly when the compressor stops and to shut off the flow when the compressor is running.

Test it! By that, I mean pulling the assembly, wiping it down, putting the part that goes into the tank into your mouth, and blowing through it.

However, some compressors do without an unloader valve entirely, putting a small hole in the line or fitting to the tank, and bleeding a small amount of air all the time. This costs the compressor owner wasted energy, yet saves the compressor manufacturer the cost of an unloader valve. Once the compressor cuts out in high pressure, the hole bleeds the air over the piston to ease the restart.

How to Test Air Compressor Check Valves?

How to test if your air compressor check valve is bad and not working effectively? I will provide you with this simple method:

First detach your check valve from the air compressor if you haven’t already, and wipe away any debris left on it. Now referring back to the image displaying the 3 ports, wrap your lips around the valve below the tank threads at port 1 and blow!

If the tank ball check or flapper valve is working efficiently, no air should exit either of ports 2 or 3. If air does leak out one or both of these ports then your check valve is not seating.

You should try washing the valve following the guidance already presented in this article. If the leak still persists, I would recommend replacing it.

I have picked out a couple of useful youtube demonstrations on how to test air compressor check valves:



How to Replace Air Compressor Check Valves

At this point, I assume that you have removed the check valve from the air compressor. If not, please do so and have a close look at it so that you can identify your replacement. For guidance, take a look at my detailed guide explaining how to replace an air compressor tank check valve. In that guide, I go into detail about locating, removing, and replacing your tank check valve for a variety of air compressor types.

Air Compressor Tank Check Valve Troubleshooting
Buying A Replacement Compressor Tank Check Valve

These are a couple of compressor tank check valves shown above. Others may vary slightly from these, yet pretty much all will have the common elements as shown in the graphic.

Have a close look at your check valve and determine where the line enters it from the top of the compressor pump (marked 3), where the line goes if there is an air line from the compressor tank check valve over to the unloader valve (marked 2), and at the thread that threads into the compressor tank to determine its size (marked 1).

As long as the replacement tank check valve you acquire has these elements in common sizes with the old, the new tank check valve should work, regardless of the brand or supplier.

Not sure about fitting thread sizes? Visit our NPT fittings and Metric fittings pages for information about figuring out the thread sizes on the air compressor tank check valve.

Replacing your Tank Check Valve with a Different Valve

It is not necessary to replace the compressor tank valve with one of the exact same make, a situation that may become necessary when smaller, lower-cost, air compressors have a parts failure and parts for it are not available at all.

It is necessary to replace the tank check valve with one that has the same operation and fitting sizes, though bushings can be used if necessary to change the size of one or more of these threads to modify them to fit.

If you use your browser to search for “replacement compressor check valve”, you will find quite a selection.

Try to find one that looks similar to your existing valve. In particular, one that has a similar thread size, and similar connection ports for the lines from the pump head and the one that plumbs over to the unloader valve.

Once the compressed air gets into the air tank that is where it should stay until such time an air tool is connected to the discharge coupler on the compressor, and air flows down the air line to the air tool. Compressed air should never escape back up past the compressor tank check valve flapper to escape out the hose to and through the unloader valve.

If that is happening it’s time either to clean or to replace the valve, and if cleaning it doesn’t work following our guide, I hope this guide on how to buy a replacement compressor tank check valve helps.

Additional Troubleshooting Pages

Air Compressor Types Troubleshooting

Air Compressor Part Specific Troubleshooting

Additional valve reading:

Reader Questions & Responses

Coleman compressor – How do you clean check valve?

Air bleeds from the compressor after charging.

How do you know if it is the check valve?

How do you clean check valve?

Checked all lines in and out of tank and compressor.

Unloader valve discharges a lot of air while compressor operating. Compressor is a Coleman.

Where the air bleeds from is important.

The first step is to have a look at the the sitemap for the pages on this site that explain the operation of the UNLOADER VALVE and also the operation of the CHECK VALVE.

“How do you know if it is the check valve?” The answer to that is described on the pages.

Also noted is the typical location of the check valve where the line from the top of the compressor pump enters the tank. Often, there is a tee off of an air line over to the pressure switch, and this is the line that goes, again normally, to your unloader valve.

It sure sounds like your check valve is leaking. Dump all the air from the compressors and unplug it before you start disassembling the line into the tank to pull the check valve, inspect it, make sure it closes, and reassemble.

Would you please provide the Coleman compressor model number and upload a photo or two of it too so we can all see what the model looks like?

Campbell Hausfeld VT558704 check valve bad

by Sam
(Plainfield, IN, USA)

Is my Campbell Hausfield VT558704 check valve bad on our shop air compressor?

Air Compressor Tank Check Valve Troubleshooting

Campbell Hausfeld VT558704 parts diagram

When the pump is off, the unloader valve is constantly releasing air.

I’ve tried to remove the check valve from the tank but the bottom of the valve is connected to something inside the tank so I can’t remove the check valve to verify that it is the problem… any suggestions?

I don’t have any idea what the check valve would be attached to inside the compressor tank. As far as I know, it shouldn’t be attached to anything

If you visit the CH website here:


…you can input the part number and download the manual and parts list.

I did so, and have added the section of the parts image relating to the check valve.

Item #20 is the check valve, and item #21 is the gasket that seals the check valve onto the manifold.

Once you have removed the air line from the check valve, compressor off and tank empty of air first of course, the valve itself should screw right out of the manifold.

I ask anyone that has this type of compressor to comment, if they have successfully removed and replaced the check valve, to help Sam out.

Check valve leaking on Emglo

by Bob Ally
(Lakeland Florida)

I have an issue with the cold start valve leaking on my check valve . I just replaced the pressure switch and the check valve on an Emglo portable air compressor and still have the same issue.

Air Compressor Tank Check Valve Troubleshooting
Air Mate Am78hc4v
Check Valve Leaking On Emglo 21868172
Check Valve Leaking On Emglo 21868172
Check Valve Leaking On Emglo 21868173
Check Valve Leaking On Emglo 21868173

Any suggestions?


Model number
by: Bob

The model is Air Mate AM78HC4V 1.5 HP. The pressure switch that was replaced is 90/125 1 port, unloader, on/off Furnas style. Hope this helps


Air Mate AM78HC4V leaking
by: Bill

OK, thanks. I’ve uploaded an image so that we all know what compressor we are talking about.

If, when you say “cold start valve leaking on my check valve” you are referring to the unloader valve leaking when the compressor is off, then it’s still your tank check valve that is leaking, Bob.

The unloader is open all the time the compressor is off. The only place air can be coming from after the pump unloads is the tank. The only way air can be coming out of the tank with the compressor off is if the tank check valve leaks.

If I’ve misunderstood the question, please clarify for me.


Check valve leaking
by: BOB Ally

Bill, Thanks for the comment, perhaps I should clarify that when I turn the compressor on the air is escaping from the small “cold start” valve on the side of the check valve.

So consequently the tank never builds up pressure.

As I stated earlier I’ve replaced both the pressure switch and the check valve. Hope this helps, Thanks


Cold start valve
by: Bill

Bob, I’m having a tough time visualizing what you mean by the “cold start valve” on the side of the check valve.

Please add a couple of photos of the check valve / cold start valve.

Once again, if, when you say “cold start valve leaking on my check valve” you are referring to the unloader valve leaking when the compressor is off, then it’s still your tank check valve that is leaking.


Cold Start leaking
by: Bill

Hi Bob…

I’m in the throes of reviewing the information you sent. Thanks.

I was not aware that this model of Emglo has basically two unloader valves, the “cold start” valve you refer to being one of them.

The purpose of both is to ensure there is no additional air-pressure load whatsoever on the pump piston when the compressor goes to start, this aiding cold weather starting.

If the “cold start” valve is leaking, the air has to be coming from the tank. If the cold start valve is truly a valve, then it’s purpose is to allow air out briefly when the compressor stops, and to shut off the flow out when the compressor is running.

So, if it ain’t the tank check valve that’s leaking, then it’s got to be a failed cold start valve. Did you replace it?


Cold start valve
by: Bob

Bill, thanks for the comments, the whole check valve is new including the cold start valve it was all one assembly. I’m thinking since it is a “second” unloader valve that I will just remove it and cap it off. thoughts?


Plugging cold start valve
by: Bill

Bob, you replaced it, I get that. Did you test it?

By that I mean pulling the assembly, wiping it down, putting the part that goes into the tank into your mouth, and blowing through it?

The other thought I had is… is this a new condition?

Some compressors do without an unloader valve entirely, putting a small hole in the line or fitting to the tank, and bleeding a small amount of air all the time. This costs the compressor owner wasted energy, yet saves the compressor manufacturer the cost of an unloader valve. Once the compressor cuts out in high pressure, the hole bleeds the air over the piston to ease the restart.

If your compressor has some years on it, meaning that parts are wearing, the fact that it’s not building pressure to cut out may not be pressure switch or unloader (cold start) valve related at all, but be a symptom of a failed pump valve or cylinder seal?

So, do you know if the “cold start” valve leaked all the time the compressor was running before the replacement?

Check valve blew on 40400 compressor

by Nathan
(Houston, TX, USA)

I have a Central Pneumatic 2HP 8 Gal compressor (40400) about 2 or 3 years old. I use it only about once a month for a few minutes at most.

It just blew what is described as the “check valve” in the manual. Part of it that actually looks like the head of a bolt actually broke and shot off.

I called the 1-800 and they said that the part was not available.

Central Pneumatic 40400 air compressor

I have looked a bit online, but can’t even see where I can buy another one. It looks like a very simple device. My guess is it is a pressure release safety valve that blows if the pressure is too high. I don’t think that is the issue. I think it was more that the part was a bit weak, but if I get a replacement I will check the pressure and make sure.

Anybody have an idea on getting a replacement?


Hey Nathan… you talk about the check valve, but then, it sounds like you may be talking about the PRV valve?

Either should be available and they need not be necessarily the same brand.

Air Compressor Tank Check Valve Troubleshooting
Central Pneumatic model 40400 air compressor

How about another post with a photo showing what it is you are looking for? Actually, upload a couple, one showing where the device was on the compressor, and another a close up of the broken device.

Check Valve for 40400
by: Anonymous Mike!

This site claims they carry parts for 40400, I have not find any negative reviews about them.

(Link removed as it no longer works – moderator)

If the link wraps, copy and paste all of it carefully, AnonymousMike!


by: Nathan

Thanks for the reply and the offer of help.

I looked at the newer model from Harbor Freight with the same HP and tank size and the part looked the same. When I got it it did have the opposite male/female threads, but I can find a fitting to fix that.

The problem I ran into was removing the old part. It shattered. as I tried to remove the mail part stuck in the female threads on the tank.

I broke one tool made to extract pipe in this situation. When I got another one I ended up denting the tank just pounding the tool in to place.

At that time I figured I had done enough damage and I had better cut my losses.

Thanks anyway.

New comment? New question? Please add it here along with photos to help others help you with your compressor check valve issues!

By Bill Wade

About Air Compressors has been helping folks with their Air Compressor Problems since 2002 online. We're a community of DIY and Compressed Air professionals who are keen to support everyone across the globe with their air compressor issues and troubleshooting. Whether you're trying to identify an old air compressor, or troubleshoot an error code on a sophisticated new industrial air compressor - the community at About-Air-Compressors.com is here to help you

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