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.
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.
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.
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.
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.
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.