I have a 25 gal compressor and it will not build pressure. The motor runs but will not build pressure. The compressor is about 25 years old

Sounds like the reed valves in the head failed. you need to replace the head (usually cheaper to get a new compressor)

the start capacitor on the motor body is weak,the motor needs this component to over come the air pressure when cycling to re-build the low air pressure,change both the start and run capacitors when doing the start because the other run capacitor is just as old and it will soon fail

The problem with compressors not building pressure is usually a bad pressure switch or bad regulator. Does the motor shut off at 20psi or does it continue running and not build pressure? If the motor continues to run with no increase in pressure, the you should go over the machine looking for air leaks from your regulator back all the way to the motor. Any air leak while the compressor is running can cause the pressure to "peak" way before the intended shutoff that it is regulated to.

The problem is lilkely the check valve between the compressor and the tank -- it is alowing pressure to build from tank to compressor preventing the compressor from turning over. Clean, maybe replace, check valve...

Charlie

worn out pump on the compressor, or leak in tank.

if they are oilless compressors then the rings are worn

If your motor is running and the pressure is not building it is either the valve plate or the compression ring. Check both for damage.

IS YOUR AIR FILTER CLOGGED. SOUNDS LIKE THE VALVES IN YOUR HEAD ARE NOT CLOSING ALL THE WAY. THEY ARE MOST LIKELY REED VALVES, THIN STRIPS OF METAL THAT OPEN AND CLOSE WHEN THE PISTON MOVES. MAKE SURE THEY ARE CLOSING ALL THE WAY (NOT CARBONED UP). DID YOU REMOVE THE HEAD AND DID YOU PUT BACK LIKE YOU FOUND IT - NOT TURNED OR FLIPPED OVER. SEE IF YOUR FINGER WILL GET SUCKED UP TO THE INTAKE HOLE.

compressor will not build pressure

sounds like the belt is slipping have you checked the inlet and discharge valves also could be bad and leaking or stuck or installed wrong