I've made very effective master cylinder gaskets from soft aluminum sheet metal or copper. All it takes is some careful drilling and filing for the inside diameter, and a good set of tin snips for the OD. They last WAY longer than paper gaskets and also seal better. Most master cylinder plugs have a small vent hole drilled into them from the side.
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!