Troublr with barrel sealing piston valve

[micquai]

The piston in the valve won't open when you drop the pressure behind it with the pilot valve. I turned the piston on a lathe put O-rings on it and it fits and moves really slick. You can blow on it and it seals and you can put vacuum on it and it moves back like it's suppose to, but when you put it all together and put the pressure to it the piston won't move back when fired. I've made a lot of air cannons with this type of valve and they always worked. I can't figure it out. Can the area behind the piston be to large? I would appreciate any help you can give me. Thanks in advance.

[Gippeto]

Can you post some pictures and provide some dimensions?

od of barrel
od of piston
pilot volume
pilot valve size and type

[jackssmirkingrevenge]

If you're filling from the pilot area, have you made provisions for air to leak past the piston into the chamber?