Compressor - Why Not Build Pressure?

What is going on when a compressor will not build pressure? The compressor runs, but the gauge showing air tank pressure moves little if at all.

Please note that an air compressor that will not start is not the same issue as one that will not build pressure so please go to this page if your air compressor will not start.

Otherwise, please read on about compressors that don't build pressure.

A compressor that starts properly, but reaches a pressure level in the tank, and then it continues to run, and run, and run, but the pressure never goes any higher, if this is your compressor problem, here are some things to check.

Compressor Power Supply

Do not underestimate the importance of a clean power supply and abundant power supply to the socket into which the compressor is plugged. Also, be aware of the size of the power cord that goes to the compressor.

If you are starving your air compressor of enough power supply, it is possible that the motor can generate a certain level of pressure in your tank, and then not exceed that pressure. Insufficient power reaching the motor will not allow it to work hard enough to build pressure against the increasing back-pressure from the tank. A too low amperage or too small power cord could be causing the problem.

If you must use an extension cord to power your air compressor, make sure it is heavy enough to feed the compressor over the length of that extension cord. Look at the power demand of the compressor motor, and make sure the cord is heavy enough gauge to deliver that power over its length. It is much preferred that you plug the power cord that came with the compressor right into the socket and not use an extension cord at all.

If you must use an extension cord rather than a longer air hose (that's the much preferred option for supplying compressed air farther from a power outlet) Google "recommended wire gauge for electric motors" and ensure that the cord you wish to use is big enough.

Clean Compressor Power Supply?

Clean power also means that nothing else that might be powered by that same electrical circuit is trying to run at the same time your compressor is. Compressors will run best with full electrical supply from a dedicated circuit, over the shortest electrical cord possible!

It's difficult sometimes to get a socket to which nothing else is connected. If that's the case for where you plug in your compressor, determine what else is on that circuit, and try to ensure nothing else is running, or going to run, when you are planning on using the compressor.

Does Your Compressor Leak?

Let your compressor come up to whatever pressure it will. If the pressure reaches a certain level and then won't go past that pressure level, then either turn it off or unplug it.

Listen while the compressor is off, and watch the tank gauge as well. What you are checking for is an air leak that you won't be able to hear when the compressor is running. If the pressure in the compressor tank is dropping and nothing is using air the leak may be bleeding off enough air that the compressor can't build tank pressure past a certain point.

Another spot to check carefully is the unloader valve.

The unloader valve should burp or hiss out a bit of compressed air when the compressor reaches it's cut out pressure and the compressor stops. The unloader valve should not have air bleeding out of it all of the time. If that is happening, this may be the reason that your air compressor cannot compress air past a certain pressure level as it is losing air as fast as it can compress it.

If the unloader valve is leaking, you need double check it but first check your tank check valve. A poorly seating or failed tank check valve is frequently the cause of what appears to be an unloader valve leak as air will bleed out of the unloader from the tank as long as the compressor is NOT running.

The Pump Valves

Reciprocating air compressors have valves that allow air to enter the cylinder area when the piston is moving down, and allow the air to be directed into the tank line when the compressor piston is on the compression stroke.

Other styles of air compressor pumps have valves too. Low cost reciprocating compressors typically have have low cost reed or flapper type valves which, at least according to the numbers of persons that report valve issues on this site, don't seem to last very long.

If either the intake valve or pressure valve fails (breaks or fails to seat properly due to debris build up) then your compressor will run all day and never build very much pressure in the compressor tank. The valve problem may also not appear until a certain pressure level is reached, at which point, the valve problem manifests itself by the compressor continuing to run, but no further buildup of air pressure occurs in the tank.

Once you have eliminated all other reasons why the compressor runs but doesn't build pressure, it may be time to tear down the pump and examine those valves.

Compressor Intake Valve

If it is the intake valve that has failed, then the compressor will draw air in on one cycle, but then that air will blow right back out the intake valve - the valve into which air from the compressor intake filter flows - and out of the pump again when the piston is in the compression stroke.

Air always takes the route of least resistance so a little of that air may actually be entering the tank, but then more flows out of the valve rather than down into the tank, which is why the compressor can sometimes reaches a certain tank pressure beyond which it won't go.

Compressor Pump Pressure Valve

If it is the pressure valve in the valve plate that is the source of the problem, air will flow into the tank through the pressure valve on the compression stroke, but then be drawn right back out of the tank as the piston cycle to try and draw more air in through the intake valve.

Again, air always follows the path of least resistance, and if it is easier for the air in the tank to flow back out through the damaged or failed pressure valve than be drawn in to the cylinder via the intake valve, then that is what the air will do.

Quick Check of Compressor Intake Valve

Remove the intake filter and feel if air is moving in and out of that opening while the compressor is running. Be careful as the pump itself can become very hot.

If air is huffing back out of the intake valve port your intake valve is suspect. For most of us, a failed reed / flapper valve(s) mean a trip to the compressor repair shop as it will mean tearing down the compressor pump to try and identify the failure, and then the search will begin for compressor parts.

Sometimes replacement valve plates are not available. That being the case, some clever compressor owners have made their own from spring steel. See the sitemap page under troubleshooting for links to pages about maing your own reed or flapper valves for compressor pumps, and also about making your own gaskets too, since almost invariably if a compressor pump head is disassembled, some gasket damage will occur.

The Compressor Gaskets Wear Out

The flow path of the intake air inside the pump head is often only separated from the flow path of the pressurized air to the tank by a gasket.

Gaskets do wear out. Good quality gaskets cost more money than cheap ones, so it stands to reason that you probably are not getting top quality parts - whether gaskets or other components - in a $40 DIY type air compressor.

It is possible that your compressor is working fine, but as it is cycling, the air is flowing back and forth across a failed gasket inside the pump insted of being forced into the tank.

Sometimes air may flow into the tank until the pressure in the tank is high enough to force an opening through a normally sealed gasket in the pump head. You need to, or have the compressor pump torn down to check this out.

If you are tearing the pump down it is advisable to change both the valve plate and all the gaskets, as taking the pump apart will likely damage the gaskets so they will not seal when the pump is rebuilt.

Economics of Compressor Repair

Let us consider the economics of compressor repair or a moment.

I am astonished as how the price of air compressors has declined over the last decade. Not the high-quality industrial air compressor, but the smaller, DIY home-use compressor.

To change out the reed valves on a small DIY type air compressor will take about one hour of shop time, according to the repair depot I spoke to. What do they charge; $60 / hour, ... more?

Then there are the parts themselves. Many of the DIY type air compressors come from another country, and sourcing parts for these units is time consuming if parts are available at all.

So, do you spend the $100 or more dollars to fix your $150 compressor or not, considering that the replacement for it may be even less than what it cost when it was purchased. That's a call you'll have to make if your compressor begins to run and run without building full pressure.

Forum / Questions About Compressor Repair

If the tips on this page do not provide the answer, tell us about it on this forum page about why compressors will not build pressure. Some of the thousands of visitors to this site have had the same air compressor issue of it not building sufficient air pressure, and will post questions and comments rich with advice.