Coaxial, piston problems

[trask_12]

I'm building my first gun, a pneumatic piston exhaust gun. It's pretty much the PAC 5 from xinventions. I've got the whole thing built but when I add air I have problems. The piston, built out of a 3" coupler sanded to fit inside a 4" chamber seals well against the barrel, and and pressurizes well. The only problem is when i open the sprinkler valve for a split second to let the air behind the piston out, the piston stays sealed against the barrel, instead of flying back and letting the air on the otherside into the barrel. My guess is that I'm not getting air past the piston since the piston doesn't move and since I can get the pressure up to 100psi in about 25 seconds of filling. Do you guys have any suggestions? Is the hardware inside the coupler - the actual piston - susposed to move at all? I sanded the piston well and it doesn't stick inside the barrel so I don't think that's the problem. I'm going CRAZY someone, please help me!!

[Benny]

did you add a equizlation hole? mabe you need a bigger pilot valve.

[trask_12]

Yes, i drilled a couple holes in both my big washers to help equalize but it's still not working properly, it seems to be 50/50 wether it will shoot or not... and when it does shoot, it's only about 250ft, when I'd charged it to 90-100psi. And for my exhaust valve, I'm using the good ol 1" solenoid sprinkler valve. Am I only susposed to let air out of the solenoid for a split second, because I've been letting it all out than like 1 second after, the piston comes back and than shoots? Maybe I'm letting too much air out?