What is going on when a compressor will not build pressure? Many of the questions about troubleshooting compressors on this website have to do with "My compressor - why not build pressure?"
An air compressor that will not start is not the same issue as one that will not build pressure. Here is information about air compressors that will not start if that is what you are looking for information on.
This page addresses the issue of compressors that start properly, but reach a pressure level in the tank, and then continue to run, and run, and run, but the pressure never goes any higher.
Here are some things to check.
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, as there just isn't enough electricity reaching the motor to allow it to work hard enough to build pressure against the increasing back-pressure from the tank.
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 cord. Look at the power demand of the compressor motor, and make sure your extension cord is heavy gauge enough 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.
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!
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 a bit of compressed air when it reaches cut out pressure and the compressor stops, but it 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.If the unloader valve is leaking, you need double check it, and also your tank check valve, as this latter compressor part is frequently the cause of what appears to be an unloader valve leak.
Low cost compressors have low cost reed type 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 as the compressor continuing to run, but no further buildup of air pressure in the tank.
Air always takes the route of least resistance, a little of that air may actually be entering the tank, which is why the compressor reaches a threshold pressure, beyond which it won't go as the flow path back out the intake valve is an easier one than down into the tank against increasing tank pressure.
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.
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 this, and making your own gaskets too.
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 $100 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. Sometimes that may not happen until the pressure in the tank is high enough to force an opening through a normally sealed gasket. You need to tear the compressor pump 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.
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; $40 / 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 $75 or more dollars to fix your $150 compressor or not? That's a call you'll have to make if your compressor begins to run without building full pressure.