You're going to find that the plug isn't sealed to start with- - - -there's a tiny air bleed hole drilled into it. The gasket is just a dust seal, and a piece of thin cardboard trimmed to fit will work well. If the bleed hole gets plugged up and a vacuum develops in the reservoir, air can be drawn past the wheel cylinder cups as the brake pedal is released after a stop.
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!