Compressor still will not build pressure

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This is page two about reasons why an air compressor still will not build pressure.

If you have just landed on this page, here is page one about why air compressors will not build pressure. On this first page we talked about the problems relating to the intake and pressure valves.

We continue from the list on the previous page with:

Possible Air Compressor Gasket Failure.

Quite often, as noted on the previous page, an air compressor will have a “valve plate” on which will be found both the intake and pressure valves. What separates these two valve areas are gaskets. When a gasket fails in the pump, the motor will run, the pump will pump, but the compressor still will not build pressure.

As the piston cycles, air is drawn in, and when the piston compresses, it’s supposed to drive the air down into the tank.

With a failed compressor gasket the air simply blows back and forth inside the pump, cycling between intake and pressure, and this limits the air getting down the air line to the tank.

Typical air compressor pump gasket
Air compressor pump gasket

The image above is of one air compressor pump gasket. Yours will almost certainly be different. The image is to give you an idea of what your air compressor gasket will look something like.

If you tear down your air compressor pump, two things to note. You may not be able to tell if the gasket has failed as often the gasket leaks when the tank pressure reaches a certain pressure, and until that pressure point, the gasket will appear to be sound.

The other is, if you tear down the pump, you will most likely damage the gasket, so be prepared to replace the gasket even if you are not sure that it’s the problem. This will save you a lot of frustration later when you find that replacing the valve plate, for example, solved one problem, but the compressor still will not build pressure.

Compressor Piston Seal Failure.

This issue is pretty easy to diagnose if your air compressor is a lubricated model and has an oil sump. Less so if the air compressor is lubed for life.

If there is an oil sump then there will be an oil fill portal, often a tube with a cap on it. If the piston seals have failed sufficiently – they are a wear item after all – while the compressor is running, air will be felt exiting the compressor oil fill tube vent cap.

Compressor oil fill tube and vent cap

While any air getting into the oil sump – and some always does –  will vent through the fill cap, a badly leaking piston seal will allow enough air to bypass the seal to create a flow of air into the sump and out the fill tube cap.

Solution? Time to tear down the pump and replace the piston seal(s). When doing so, it might be a good time to replace the valve plate, depending on how old and used the compressor is, and, of course, the gasket kit as well.

Tank Check Valve is Compromised.

As air flows from the pump head, through the air line, and into the tank, it passes through the tank check valve. The tank check valve is normally located where the line from the pump enters the compressor air tank.

Location of air compressor tank check valve

The photo above shows where the tank check valve is located in typical air compressors.

The tank check valve, like most of the components on the imported air compressors of today, is not a high quality item, normally containing a simple flapper type valve resting on a seat.

When air from the pump enters the tank, it blows this valve off seat, and air enters the tank. While the compressor is off, the flapper valve is pressed onto the seat by air pressure, sealing the air in the tank.

If the check valve fails, it may prevent air from flowing into the air tank properly. Or, if the check valve has failed, compressed air won’t be staying in the compressor tank when the air compressor is stopped, it will vent out the unloader. That being the case, a new check valve is in order.

Are you still having an issue with an air compressor that still will not build pressure? Questions and tips for other compressor users can be posted just below. Thank you.

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Jack Costella
Jack Costella
March 27, 2017 12:15 am

I bought a $100.00 140lb V2 pump, as a hold over until I repaired my 2stage . It pumped to 140# for 3 weeks Then kept building less it pumps 40# now. Pistons ,rings are beautiful .valve plates are flat, .010 reeds are flat..Both outlets really (pop) I have it pumping into a small propane tank to test. a large frame 220 motor , the pump turns 1050 RPM both intakes really suck.
I just don’t make sense. Has anybody had similar problems?
Thanks Jack

Fix My Compressor Moderator
Fix My Compressor Moderator
Reply to  Jack Costella
March 27, 2017 10:01 am

You might want to check the motor capacitors, Jack.

Steve Grayson
Steve Grayson
April 27, 2016 10:51 am

Had problem with my Campbell Hausfeld portable compressor not building pressure. After much effort checking the various suspected parts, my frustration was leading me to the store to purchase a new compressor. Decided to do one more analysis with the protective cover removed and turned it on and ran my hand above the various parts and felt air blowing out somewhere between compressor and check valve. Turned out it was the exhaust tube had ruptured and that tiny crack was venting all the compressed air before it could get to check valve. Hope this might help some of you dealing with similar frustration trying to diagnose same problem.