I have replaced the shaft and flapper with stainless steel parts running in brass bushings. I make the bushings from 1/8" NPT pipe nipples, threaded into the manifold and line-reamed so the shaft runs true. Lots of work, and fairly expensive, but that fix lasts about 3 days past forever.
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!