Put a vacuum reservoir canister somewhere. I've used discarded Freon canisters, or even a length of big PVC pipe with both ends capped, bracketed inside a frame rail. Fit a grommet into one end cap that will accept a check valve for a brake booster, and run a line from another fitting to your booster. The tank will stabilize the vacuum and hold it anytime the manifold vacuum gets low.
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!