It's been 30-something years since I was involved in medium and heavy truck work. It's a pretty simple task to mount a reservoir tank or two inside a frame rail and run a vacuum line from the intake manifold to it. Then attach lines going to the brake booster and/or the axle shift system to the tank. Since the vacuum is trapped in the tank by the check valve, there's a reserve of vacuum available even if the engine stops. It's simple common sense that any plumber could figure out. On smaller trucks like a pickup or a 1 ton, a 3 foot length of 4" PVC drain pipe mounted under the cab with a couple of end caps glued on accomplishes the same sort of vacuum reserve.
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!