The valve is not original. It was purchased it a kit. The piston seals OK using same size o-ring, but then again, I can't accumulate enough pressure right now to be 100% sure.
Funny you mentioned the screw hole - I was thinking the same thing because I was so sure and careful with the valve install. But here's another funny thing. With the screw completely removed, the piston will suck air into the airtube thru the screw hole on the intake stroke. I'm thinking now it's got to be the o-ring, but I don't know why (if you don't know why it could happen again, to me anyway).
I know I'm gonna have to tear her down again, was dreading this.
Wanted to know if there is a fail safe procedure to avoid this leak