1 hp 3gal craftsman compressor will only build up to 20lbs pressure in tank. how much compression should single cyl have on compression stroke?

Might be the valve plate is worn or has a broken reed.

Is the petcock down below open or closed? If open, then screw it closed. Also, is the compressor portion connected to the motor working as it should? As the motor reciprocates, it should operate the compressor piston that pumps air pressure into the tank. Maybe something is broken there.

Sounds like the compressor has failed, more than likely a scored cylinder wall.

sounds lik crank is broke on pump pull off head dat will tell u wats wat

That small piece of rubber came from the compressor pump, more than likely you have a broken valve or blown head gasket in the pump, that is why it will not build pressure.

your pressure adjust valve is stickn unless your bottom valve isnt shut all the way

Without knowing specifics of your unit, If there are three reeds, then two are probably for intake and one for exhaust. Intake reeds should be on the piston side of the valve plate, discharge reed should be on the head side.

There should be a divider in the head side to separate the inlet from the discharge. Make sure the valve plate is turned so that the air from the air filter side goes to the side with the intake valves, and the air from the discharge valve should go to the side with the pipe to the tank.

Does the tank have a serious air leak? If it looks like it is centered around the compressor, remove the cylinder head. Check and clean the intake and outlet valves. Check the safety valve on the tank that it is not leaking. Listen everywhere for a hiss while it is running. Make sure the belt isn't slipping.

check this blog for assistance. reed valves or piston worn

http://www.fixya.com/support/t1104466-craftsman_air_compressors