Surprisingly enough, NAPA stores can usually order a rebuilt hydrovac unit for most of these old trucks- - - -they're still in use enough for the rebuilders to keep turning them out. They're not cheap, but having good brakes set up the way they came from the factory is good insurance- - - -especially in case of an accident when some ambulance chaser lawyer starts questioning why the brake system was modified as he sues you for everything you own.
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!