well, here you go. i revamped mine. i found that using rounded grooves for the o-ring doesnt quite do the job, once you get to high pressures the oring will slip out of its groove... generally just being a pain in the buttock. the grooves need to be square. also, what i found along the way, make sure you bore out one solid piece of metal on a lathe. cut it in half. 2 pieces at one inch is a nice size to work with. i used 1/4" aluminum rod, and threaded it to 1/4" fine thread (28 tpi). and very important!!! make sure you cut the o-ring groove as close as humanly possible to the face of the piston. this way you get maximum air transfer to the valve. now, if you bore a hole all the way thru, make a short plug to screw into the face, seal it with jb weld. once all is done, polish the crap out of the sealing surfaces. and for the back end of the piston where it connects to the linkage... just copy your original piston. once ready to install, threadlock one half of the piston, adjust to where you think it should be and try it out. you want the face to press against the valve slightly, enough to lock the pump arm. once you have your length, measure the distance between the two halves, threadlock it and throw it back together. you got a metal piston.
i also suggest installing an oring close to the pump linkage, for alignment purposes. otherwise your piston will scrape on the inside of the tube, creating brass powder... i just use a couple wraps of electical tape. also, you'll probably want to cut a groove just behind the oring to put some yarn, or foam or felt.... something to keep your gun oiled.
well, heres a pic, with all the good measurements that i used
enjoy
any questions, feel free to ask... oh yeah, of course... you'll have to cut the cone off the front of the valve. remember, as close to the oring as possible without losing the lip. mine is so close it can cut paper... but i dont suggest going THAT close.
maybe i'll chrony it again 
j/k. man, if ihad a shop id be livin in it!