Most, if not all hydrovac systems have a vacuum reservoir tank mounted somewhere inside a frame rail with a one way check valve between the intake manifold and the tank. The tank stores enough vacuum to allow one or two brake applications if the engine stalls for some reason. If there's a vacuum shift two speed rear end, that same vacuum tank makes axle ratio shifting smoother.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!