I understood that part. The main thing I was not understanding was the piston. Plus I didn't want to clutter this thread too much. But I guess I'll just explain both possible scenarios.
It basically sounds like you want to make a tee valve with the port that the piston sealing against being the same size as the pipe it slides in.
In the case of a chamber sealing. It would work to an extent, being that it would let air out of the chamber. But how fast it would do that would be a different thing. The piston will start moving as soon as you start to vent since the surface area on each side is the same, but once the piston cracks open, the pressure on the chamber side will quickly fall, and often the pressure differnential will not be enough to continue to move the piston. I have tried a valve like this http://forums.spudtech.com/topic.asp?AR ... IC_ID=1400
and it simply cracked open a little and let all the pressure out. Notice on the animations that right when it starts to move back the force acting on the piston makes a jump, but you don't have that jump on this piston design so it will tend to have some problems.
The other thing it could be is a barrel sealing configuration, pretty much what MonkeyDoo was trying to describe earlier in the thread. In this one you don't even need a pilot simply because pressure back there will do nothing except maybe shoot the piston out the barrel if you don't have a stop for it. But you do need something to pull the piston back. In theory this will work because you do have that force jump. I don't think I have seen a gun that relies on pressure to do the work once the initial seal is broken, but the Big Red cannon does use this type of piston with a pneumatic ram type device to pull it back. And it seems to work quite well. The main thing wrong with this design is that it is a safety hazard. Some pressure leaking into the barrel side or a slight bump could fire the gun accidently.