We use two metal tanks tied together with 1" braided hose to a Metal ball valve that is manually operated (it is the safety valve). Out of the ball valve we use a 1" sprinkler valve actuated with a spike relay and then to some braided hose to a 4" PVC barrel. We operate the whole system at 60 psi and have to repressurize the tanks after 4-6 shots.
Yes, the plastic sprinkler valve is not rated for air, but it was simple and with the safety valve, it isn't held under pressure for that long. We've used the system since 2004 on 4 different drive bases. Our current drive base has articulated mecanum front and back.
Here's a quick video we did for the local tv station website...
www.vandenrobotics.com/Videos/RobotSpirit2.wmv
We can launch up to 4 cheeseburgers to the top of our stands at games.