I was looking at the one last night. I just can't pull the trigger on it yet though, until I know my current piston is the weak link. I think for now I will either leave the vacuum piston as is, and move on the the hopup / cylinder head seal, or try and do a non permanent mod to the air release valve in the vacuum piston. Fill the hole with an epoxy or something, not sure yet.
EDIT: OK, tonight I decided on a semi permanent solution to the vacuum piston leaking air. I used some steelstik, filled the chamber that houses the little rubber ball in such a way as when I screwed it back together (after it had cured) it forced the rubber ball deep into the intake hole. So even if the steelstik doesn't create a perfect seal, the rubber ball forced further into the hole will (in theory). I have 100% compression in the cylinder now, no matter how long I hold it. Good stuff!
I have used steelstik before, but never for applications where there is pressure and lots of pounding abuse. It will be interesting to see how it holds up to the punishment of existing in a cylinder with a 430fps spring.
On to the pdi hopup unit with the two potential leaks, cylinder head to hopup bucking and bucking to barrel. I cant find ABB devices for sale anywhere, and not too excited about trying to cut my own, but I do have a few ideas (will try the floss or wire mod).
Also, have any other maruzen type 96 owners noticed when the outer barrel is screwed all the way on, it's not in alignment with the receiver? I'm thinking some careful sanding down of the barrel edge (with the threads) can solve this, but I'm a bit nervous to try it just yet.