rebuilding an air compressor?

[misk]

today was the first time i used the air compressor i picked up for $20. it has a 40lt tank and i was use a die grinder at first.
now i wasn't expecting much, but when i replaced the oil and then saw a stream of oil squirting out of the oil res. i thought something was up lol.
if i put my finger on the breather hole in the oil cap it builds pressure faster, so im guessing the piston doesn't seal properly anymore.
it also seems to struggle keeping up with just normal use of air tools. so im assuming something is up inside.

has anyone ever pulled one apart to reco?
any idea where i'd be able to get spares n shit for it?