Finally did a full teardown and found out what the deal was. The previous owner had done some seal replacements by himself, didn't seem like he did too much damage, but what he did was a silly mistake. Obviously there's a setscrew that holds the valve in place. He overtightened it, and it prevented the valve stem from going all the way through the valve, hence the constant valve leakage no matter what I did, and not being familiar with the valve dimensions, I never noticed. I SHOULD have noticed the lack of protrusion of the valve pin in the hammer/bolt carrier section of the tube though. Bah, lack of tinkering experience. I'll get better. Of course I had to remove the valve set screw all the way and the screw was so old and gunked up with paint and what I would assume is blue loctite (why that was there, I don't know.) that I wound up having to twist real hard to remove it and stripped the threading out :-\
Do you guys know the appropriate thread sizing? Maybe my dad's got the tap in his box of toys.