If it leaks down the barrel when it's cocked, you only have the valve poppet seat and the valve body o-ring to look at. They come out at the same time, when you separate the body halves.
If it leaks only when the hammer is forward, you have one or two things out of whack.
I forget the size of that o-ring. It's about bottle/ASA size. I think sugarstump and I&I still have rebuild kits.
That o-ring is not a common leak. It's static and held by the body halves (they're not loose right?).
If it's a small leak, try some oil in the ASA and shoot it through. The poppet may just be dirty.
Last edited by Spider!; 11-28-2012 at 05:33 PM.