Pneumatic Piston valves.

Hallo, im fixing to build a Pneumatic spud cannon out of galvanized steel. i have pretty much everything that i need, but I seem to have hit a wall in trying to find one thing. I need at least a 6" diameter piston valve capable of dumping the excessive amounts air within two rather large compression chambers. While the supah valve on the Spudtech site is indeed impressive, i'm afraid that it won't nearly be capable of dumping as much air as the cannon will have in an efficent way, let alone capable to withstand the pressure.
I would greatly appreciate anyone who might point me in the right direction towards obtaining a valve which might suit my needs.
Any insight into other aspects and mechanics of the cannon would be greatly appreciated as well =o).

Thanks a bundle.
-charlie