I'd like to help but I might not be right. I'm just making suggestions of what I would do and hopefully prevent you from buying new parts. Start from the source and work out ward to the barrel this way you know where you have been.
Compression unit
Teflon tape the cylinder head. I like to wrap mine so that once in place the tape is half in the cylinder and half out. Then fold the protruding half back over the cylinder to sandwich the tape between the cylinder and head.
Check your o-ring on your piston. I like #14 o-ring from hardware stores for normal non-boreup piston heads. But if the metal piston head has a wide gap you might have to take it with you. You might want to try a new air nozzle if you find that it is the weak link. Deep fire air nozzles fit particularly tight and their cylinder heads may be a slightly different due to this.
Parts (possible)
Deep Fire cylinder head,
http://www.deepfireairsoft.com/airsoft-piston.html
Cylinder, there are many companies that use ribs including Deep Fire. Could this be a bore-up?
Piston, sounds like Modify or Echo1 pistons,
http://www.modify.com.tw/products_e.html
Tappet plate, Classic Army,
http://www.classicarmy.com/search.jsp
I hope this helps, it sounds like it could be about anything. My best advice is to start at one end and just rule'em out so you don't go crazy.
good luck