Okay....despite the leak that continued to develop, this gun just kept giving me more and more problems
Here's what happened: I pumped it up to around 60 psi and a new leak developed
so I stopped pumping and fired it immediately. The pressure, though not as high as I have taken it on this gun before, was enough to rip the neoprene washer out from BEHIND a steel washer that was supposed to hold it in place and was shot out of the barrel
I've decided to take what I've learned from these mishaps and make a brand new valve with the same piston, though I have used 2-part epoxy on the washers to hold them together and I have also tightened them down much more than last time. I'm also using the HD pvc cement, for use with fittings up to 12"!! (as opposed to 6") Hopefully, all will work well....
Anyway...here's a pic of the gun before I cut the valve open to extract the piston (this is what it will look like when I'm done with the new valve, as well):
WISH ME LUCK!!!!