It's possible to set up the master cylinder in a lathe and bore out the casting for a sleeve. That requires some pretty creative setup work to get the bore hole parallel with the spindle so the hole can be bored accurately. Sometimes a special mounting fixture needs to be fabricated first. Once the hole is enlarged, then a piece of seamless stainless steel tubing is pressed in, and bored and honed to the proper diameter. If the master cylinder has a blind hole with out openings on both ends it's a bit more difficult to machine it, but not impossible. The fluid entry and vent holes can be drilled by going through the fill hole before final finishing of the bore. It's a pretty long and tedious process, but the stainless liner should last 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!