FighterAce's thread here inspired me to create my own thread on the check-valved piston I've been working on for quite some time now, and it seems like I've finally got a good chance of success. The idea here is to create a piston valve where there is no leak from the chamber to the pilot during operation.
There is a 1/4" OD o-ring between the 6mm BB and the threaded rod, I just didn't happen to include it in this picture.
Note the fail that occured as I cut the rearmost o-ring groove. Turns out the hole for the valve was too off-center:
Right now my greatest problem is that my 1/2" UHMWPE rod that I purchased from McMaster is actually >1/2" in diameter. Hopefully I'll be making some significant progress soon, as I currently have no operational pistons.