In Big Transit, we used to have a smaller solenoid mounted in the engine room that had no bottom on it.
A pencil pushed upward into the bottom activated the system.
They retrofitted covered ones, due to the safety issues of the trick bypasses the starter inhibit for the transmission being in neutral.... Or some fool would try to start it with the ignition still turned off, emptying the tank...
Far away and long ago... has me wondering, was that a smaller solenoid that in turn activated the big valve pneumatically? There were only small lines leading to it...IIRC. GM 5307's from the factory.
Oh the many things I wish I'd paid more attention to earlier in life's experiences!
happy coaching!
buswarrior