Inline sprinkler valve is the answer for quick dumping of the tank. They work magic. We purchased plans from
http://www.americanaircannons.com/plans.html for building a t-shirt launcher (yes its called a "launcher", not gun, shooter, or cannon, the school doesn't like that). The main bit of the project is a 3/4" or 1" in-line sprinkler valve, which has a manual control on the back and the solenoid (which is 24V AC) oddly works with 12V DC off of a spike.
Anyway, here is ours, attached to our 2005 rookie bot:
