At the level of OCD you're showing, I'd suggest using a chucking reamer in a Bridgeport mill or a VERY accurate drill press to enlarge the vacuum piston bore, then fabricating a piston from soft aluminum like Jon suggested, one that's a very tight slip fit in the new, perfectly-sized bore. A few thousandths' increase in bore diameter will also allow much more sensitivity to small levels of vacuum to keep the power valve closed.
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
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