Exhaust valve for piston cannon

[Shrimphead]

I am going to be building a piston cannon of my own design. I need some ideas on some exhaust valves that are cheap, but let a lot of air out so the piston gets sucked back quickly
Thanks

[boilingleadbath]

PVC ball valve, attached via a 45* elbow. The handle is extended to about 1.5" radius, and has a hole in the end through which a string is tied.
Pull the string strait back to fire... works like a charm for me. (though I spent the extra couple dollars and got a brass ball valve)
Total cost for 1" exaust valve: like 6$.

The 45* angle is an improvment; having it mounted strait require me to pull at an angle, torqueing the launcher off to one side.

[CS]

Were you planning on remotly firing it? A sprinkler valve (modded) is your best and fastest way to exhaust air that you can get at a store. You could also remotly fire a sprinkler valve also. Never done it but im thinking that you could just wrap a string around a blowgun trigger that is if was attached to the sprinkler valve via a close nipple (no hoseing). Plus you wouldnt need to mount the sprinkle valve on a angle either. Sprinkler valves are more expensive (sprinkler valve/ 1/4"close nipple/ blowgun > ball valve/ 45 or 90)

[Shrimphead]

Blowgun????
No, im not planning on firing remotely, but thanks for that info.
Boilingleadbath, Im kinda slow and am having trouble picturing what your telling me. A quickly sketched pic would really help
Edit
I reread what you wrote and i understand it now. The only probably would be that having to pull back on the string like that would cause the gun to move and it wouldn't shoot where I was aiming, right?
It has to be easy to shoot because accuracy would be needed. Also, to help with that, on spudtech, Joel (is that right) is selling rifled barrels.
Good idea?
Thanks

[CS]

Worthless trying to explain anything Ill get a pic for you because you obviously dont have a clue what I was talkin about.