It's a sure bet that the rings are gone and piston blow by is making the oil foam out as others have eluded to. My central pneumatic manual says to call harbor freight @ 1-800-444-3353 for parts. I found your question on other forums but no source of parts for central pneumatic compressors beyond replacement motor control pressure switches, they are basically cheap Chinese throwaway units.

Sounds like you have a compression leak around the piston, in turn pressurizing the crankcase and blowing oil out the cap.

check the valves connection.Some times if a compressor machine pressure occurs high then leak it out.

The rings on the piston are worn out or broken. Remove the cylinder for inspection and repair.

it starts up and runs but it won't build pressure past 30 psi and after a few minutes it will start to leak oil out of the oil cap.. could anyone help me and let me know what I need to do to get back to normal. Please and thanks! (: