I never saw the family rigs, just heard about them. It's probably an aftermarket accessory, and rebuilding will probably involve fabricating home-brewed seals. I'd suggest making an aluminum piston that's a close fit to the cylinder bore and grooving it for 2 or 3 big O rings. Valving the vacuum to the cylinder can probably be done with air brake components of some sort. Be sure to include a vacuum reservoir tank somewhere in the system to get at least one power-assisted brake application after the engine stalls.
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!
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