[EricH]

It's that little brass valve with a not-quite-triangular twist handle on one side. If the handle is parallel to the tubing running into the valve, you either have no air in the system, or will have no air in a couple of seconds. If it's at right angles, you'll have as much air as you are legally allowed to have, or you're filling the system. (Be warned, you don't want to have it open when you're trying to fill the system before a match. It's bad...)