My 1 hp. 12 gal. air compressor is not building air pressure the motor runs the pressure won't go above 20 lb.

It could be that the input air filter is badly clogged. I might more likely be that the Non-return valve on the air tank input is not sealing properly and not allowing the tank to build up pressure. This valve is at the end of the main pipe runs between the compressor and tank. It could also mean (less likely) that the poppet valves inside the compressor cylinder head are not sealing properly. Other faults could be worn rings on the compressor pistons or a bad seal between the compressor cylinder head and block.

To check the Non return valve allow compressor to build up some pressure and then loosen union nut on Non return valve. There should be some air released at first but should stop if valve is ok. If leaking remove valve and check for dirt and clean. If valve still leaks replacement is the only fix.