Mine leaked at the rear of the air cylinder. Easy fix...unscrew the end-cap, slide off the shroud, empty the air cylinder and simply unscrew it (Lefty Loosey). There's a big O-ring between the base of the air cylinder and the gun frame. Just pop it off and replace it. Don't look for nicks, breaks, flaws, sometimes they simply don't show up to the naked eye!
Not being one to get all picky...I grabbed a similar sized O-ring from my Harbor Freight red box of "O" rings, which seems to have every size there is, and slapped that sucker right on. Screwed her up nice and tight. Been months now and not one problem keeping air at 3400 psi, with weeks between use.
I was going to replace the tiny o-ring behind the air gauge, but couldn't get a socket to fit in there, so I left it alone and no problem. (How DO you get that gauge out???)
Regards,
Kindly 'Ol Uncle Hoot