Since we don't know what compressor model you have, usually the piston/cylinder is similar to a standard gas engine and with a gas engine you'd have to pull the cylinder head off, then the jug or cylinder. Once that's off, you should have access to piston and/or rings. If you go that far you should check the condition of all those parts like the condition of the piston, cylinder walls, etc and replace them while you're in there.

Easy to remove the cylinder and piston/driver. Remove four head bolts and lightly tap head to release gasket. Push out driver by inserting rod thru nose then remove spring retainer holding the cylinder in place.. With retainer removed the cylinder should pop out. Sometimes the retainer will cause a bur to form on the body of the mailer making it s bit more difficult to remove. Good luck