Why is it a compressor will not build pressure? Does the following describe what’s happening, or rather, what’s not happening, on your compressor?

The motor runs and runs. The pump sounds like it’s operating, but the air pressure in the tank is not rising in pressure at all. Or, the pressure in the tank rises to a certain pressure level and then the pressure stops rising, even though the compressor continues to run and run.

Leaving you wondering why your compressor isn’t quite working how an air compressor should work!

I will provide you with the the most common reasons as to why your compressor is not building pressure, the severity of them and provide you with ways to check each one.

Table of Contents

Why is my air compressor not building pressure?

The air compressor is really a pump driven by an electric motor, or perhaps with a motor driven by other fuel types like gasoline or diesel.

The part that actually compresses the air is often referred to as the pump, or compressor pump.

Cylinder And Pistons
This is an air compressor pump, though it’s missing the pump head cover in this photo.

In the photo above, part of what’s missing is the pump head, which will contain an intake port. This pump is also missing the valves or valve plates which open and close depending on whether the pump is in the intake or compression cycle.

On all air compressors, the pump pulls in free air from the atmosphere through an intake valve. That intake port on the pump typically has a filter on it to keep dust out of the pump. The pump is then supposed to drive the air it takes in into the tank, and in so doing increases air pressure in the compressor tank.

Different styles of air compressors, reciprocating versus rotary screw for example, accomplish this with different methods, yet they pretty much all do the same thing, suck in air from around the room or through an outside air intake and drive that air into a tank to build up pressure.

When a compressor pump is driven by a properly working motor, and the pump is cycling, why won’t the pressure in the tank build? Next comes the most likely reasons.

What are the most likely reasons for my air compressor not building pressure?

The most likely reasons for your air compressor not building pressure are:

  • Compressor intake valve failing
  • Compressor pump pressure valve failing
  • Compressor gasket failure
  • Compressor piston seal failure
  • Tank check valve is compromised

Compressor Intake Valve Failing

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.

How to check if the Compressor Intake Valve is the Failing

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 making 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.

Compressor Pump Pressure Valve Failing

If it is the pressure valve or pressure switch 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.

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 tank pressure level is reached, at which point, the valve problem manifests itself by the compressor continuing to run, but no further buildup of air tank pressure occurs.

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 Gasket Failure

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 instead of being forced into the tank.

How to Check if the Compressor Gasket is Failing

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.

Compressor Piston Seal Failure

The compressor pump sucks atmospheric air through its intake valve and has a filter that aims to restrict dust particles from entering the compressor tank. The rings on the compressors pump act as seals, which prevent excess oil from being able to flow into the compressor’s cylinders.

However, another common issue for the air compressor not building enough pressure might be from its pump’s defective rings. If the rings are defective, the pumps will lose pressure and compression strength. This can also reduce the pump’s efficiency to optimise the production of compressed air.

How to Check if the Compressor Piston Seal is Failing

It’s worth checked the piston seal and rings at the same time. Normal checks on air compressors conducted by technicians include rebuilding or replacing the pumps with worn seals to confirm that both pumps and rings are operating efficiently, the valve plate seal might then be the source of the leak.

An alternative way of detecting pressure loss from piston rings is when air or oil is coming out of the oil fill tube. Regular inspections on air tube fittings, ensuring they are firmly attached to their rightful positions must be conducted. It is possible however, that the piston ring failure causes other components inside of the cylinder to be damaged.

If the piston cycles with worn down rings, metal-to-metal contact is quite inevitable. This is a very common cause of pressure loss within the combustion engine with a stuck air compressor piston. This fault makes it relatively easy to notice a lack of pressure build up, as pressure will not be building up from the gauge.

Tank Check Valve is Compromised

The check valve is an important component that restricts compressed air from back-flowing into the pump.

When this component is defective, the discharge head of the pump will receive a high pressure of air. It will affect the pump’s motor might and stop it from restarting anytime the tank is full. However, the motor will start when the air tank is empty.

How to Check if the Tank Check Valve is Failing

If your air tank has a defective check valve, there will be air leaks from the unloader. So you must run another spot to check carefully on 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 the air will bleed out of the unloader from the tank as long as the compressor is NOT running. Try to confirm if the air leaks continue when the pump is inactive, and replace the check valve immediately.

Summary of the Most Likely Reasons for my Air Compressor Not Building Pressure

Below is a summary table of the most likely reasons for your air compressor not building pressure with the severity of each and cost implications provided:

ReasonSeverity (low-mid-high)Cost Implications Cost Estimate
Compressor intake valve failingLowA replacement valve + 1 hour labour$120
Compressor pump pressure valve failingLowA replacement valve + 1 hour labour$120
Compressor gasket failureMid A replacement gasket of a good quality (more costly) + 1 hour labour $150
Compressor piston seal failureMid Replacement piston rings + a good few hours labour to disassemble the pump$300
Tank check valve is compromisedLowA replacement check valve + 1 hour labour$120
Summary of the Most Likely Reasons for my Air Compressor Not Building Pressure Table

The costs are estimated on a rough labour cost of $80 and then the cheapest parts available on Amazon. It’s important to note that if you wish to buy the best parts that your costs will increase, and that if you can do the maintenance yourself, you will of course save labour costs.

Low severity indicates a quick fix and your air compressor will be back up and running with no subsequent issues. High severity indicates a longer fix with damage done to other components within the system.

What to check on an air compressor when the compressor will not build pressure?

Okay, so I’ve provided you with the most likely reasons for your air compressor not building pressure. Now, let me provide you with a more general check you can conduct and some other potential reasons.

Compressor Valve or Gasket

The air compressor reed or flapper valves (also known as the intake and pressure valves, or suction & discharge valves) are a common cause of why an air compressor will not build pressure.

The image below shows the valve plates (top of the photo) and reed valves (bottom left of photo) from one smaller air compressor. It also shows the gasket (bottom right in the photo).

Compressor Valve PlateCompressor Valve Plate Kit

If the compressor valves or gaskets are what the pump problem is, then it will be necessary to disassemble the pump to repair that problem. The issue is in determining whether it is the valves that is the problem.

One clue to a prospective valve problem is if air is exiting the intake port when the compressor is cycling. Removing the intake filter briefly will help determining this. If air is escaping there it’s a pretty good bet that the intake filter flapper is damaged in some manner.

Pressure Side Reed Valve or Pump Gasket

If it’s the pressure side reed valve, or if the pump gasket that is creating the problem, that’s harder to diagnose, yet the symptoms are the same. Either no pressure builds in the tank or the tank pressure increases to a certain point and then stays there, regardless of how long the compressor runs. That might even result in the compressor shutting itself down if the motor overheats.

The only real check here is to pull the pump head off and examine what’s what. If a reed valve is damaged, that’s usually pretty obvious. If the pump gasket its weakened and blowing open at a certain pressure, that’s harder to see.

The solution to the latter is to replace the gasket regardless of whether the reeds are damaged or not.

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.

Compressor - why not build pressure? Power supply is an issue.
Compressor Power Supply

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?

If it does, 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.

Compressed air leak - compressor - why not build presure
Air Leak

Economics of Air Compressor Repair

Let us consider the economics of compressor repair for a moment, I provided with a rough idea of the severity and cost implications in the earlier summary table.

I am astonished as to 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; $80 / 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 without building full pressure. It may be worth investing in a brand new compressor.

FAQ’s (Frequently asked questions)

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 necessarily the same issue as one that will not build pressure. There is a page on this site dedicated to air compressors that will not start, and it’s linked from the troubleshooting page, if that is the compressor issue you are facing.

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 more things to check.

Why does my air compressor take so long to fill up?

Usually, if an air compressor is taking long to the fill, it typically is either a piston seal problem, gasket failure, or an intake or pressure valve failure. These issues can be checked for by following the information provided in this page.

How do you increase air pressure in a compressor?

To increase your air pressure, so that the air compressor kicks on sooner before the pressure drops too low. You can adjust the lower adjustment screw on the pressure switch by following these 4 simplified steps:

  1. Remove the plastic cover over the pressure limiter switch
  2. Locate the two limiter switch . Tun the upper limit adjustment screw to raise the limit
  3. Turn the compressor on and adjust the screw until you reach your desired pressure on the gauge. To maintain this pressure- turn the lower limiter adjustment screw clockwise to raise it so that the compressor kicks on sooner.
  4. Connect your tool and charge the compressor, test the limits you have set and make adjustments if necessary.

It is important to ensure you consult the manual provided by the manufacturer or contact them directly to find out what your upper pressure limits are so that you do not exceed its safety limits.

How do I prime my air compressor?

To prime your air compressor you must add oil to both your receiver and the suction port of the compressor. It’s important to know the oil capacity of your compressor before starting, then begin with half being poured into the receiver and half into the suction port.

If you have an oilless compressor, you must fill the pump with fluid that acts as a coolant, to help prevent the pump from overheating. The fluid will help remove the air and stop the compressor from running dry, which may lead to mechanical seal failure or even serious damage to the pump’s components.

How long does it take for an air compressor to build pressure?

It can typically take anywhere in between 2-5 minutes for an air compressor to build up pressure. It shouldn’t take any longer than this. The greater the CFM of your compressor the quicker the compressor can pump air into its tank. Also the larger your tank, the longer the time needed to fill it due to the increased volume.

Why does my air compressor keep running?

If the compressor motor is still running after the air pressure in the tank reaches the normal cut out air pressure level, then look to your pressure switch first as the source of the compressor problem.

If your air pressure in the tank is not reaching the cut out pressure limit then, the compressor cannot shut off and it will keep running.

You must look to the intake valve or an internal gasket leak as this possible source, visit our page Air compressor will not shut off for more information.

What controls the air pressure on an air compressor?

The pressure switch controls the air on an air compressor by determining how much air pressure is needed to run a pneumatic device that you wish to connect.

The pressure switch is the most basic and important setting on your compressor.

Get some help with a compressor that will not build pressure!

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Please provide as much information about what your compressor as possible. The make, the model, the age of the compressor, how much it is used, the type of power supply… anything that you can think of that will help us (myself and the many visitors) help you with this air compressor problem.

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