The compressor runs but the pressure will only build to approx twenty pounds and the compressor keeps running. I have used it many times before and had the discharge regulator set to approx 45 pounds, and was pumping up my jeep tires to 34 pounds when I heard a change in the sound of the compressor and the pressure would not go above approx 20 pounds.

The compressor will run stop start or load unload. There is a switch for this. Make sure the right mode is selected.stop start for light use. Load unload for heavy. This mode keeps the starting and stopping to keep from burning up the motor. That is rated by stops per hour. They shouldnt be more than 10 times per hour

Does it have an intake air filter? Has the compressor been properly maintained? Oil less? It may have worn out rings and or valves

If your compressor keeps running and only builds to 25psi, I would suspect the reed valves, head gaskets or a leak. With the compressor running spray soapy water on all fittings, around the head area and tank components. If no leak is found, remove the air filter and feel for suction at the intake(make sure head is not too hot). If you feel good suction at first then suction is lost after reaching 25 psi, most likely reeds or gaskets (with bad reed valves, sometimes you will actually feel air blowing out of the intake). Good luck

sounds like the winding

sudden loss of capacity like that usually indicates broken or leaking valves in the head, or a blown gasket between inlet and discharge side of the head. if nothing is found there, then look at the piston and or cylinder bore for damage.

It looks like there might be a knob which sets the max air pressure coming from the compressor. If there is one, this may have been turned down.

There are usually 3 pressure settings on an electric compressor. Compressor max, pressure switch cut on, and pressure switch cut off.

sounds like your check or reed valve is bad. when the piston pressurizes and pushes air into tank the pressure should not be able to come back into cylinder and thsi is what its doing and then that pressure is now going past rings into oil. look for the valve before the tank. a check valve.

compressor will not build pressure

Check the pressure switch, they can become stuck in the open position. You can jump it to see if it is functioning or use a multimeter if you have one. Assuming that you've oiled and taken care of it it is unlikely that it simly quit, but that could happen... check across the poles of the motor for continuity which will tell you if the electric motor is blown. ALso make sure the compresor is not seized...

Not sure of what cut out refers to. If you mean the power supply is tripped out after twenty secs hen I would say there is a fair chance of the compressor being down to earth (ie broken). If you mean that the compressor tries to run and then stops after twenty secs then there is a fair chance the compressor is stalled (ie broken). There is also a chance that the relay (starting device) may be at fault failing to start the compressor. If it is the relay then the cost may be around £70 if its the compressor then you are lookin at a couple of hundred pounds approx. The buzzing sound does sound like the compressor trying to run but stalled and unfortunately compressors have no real average anymore and are prone to fail whenever they choose to. Sorry to sound negative and hope this helps.