The rule of thumb has always been to keep the fluid level at least 1/2" below the level of the threads to allow for heat expansion of the fluid and/or splash from bumps, etc. Brake fluid is soluble in alcohol- - - - -use a spritz bottle to soak the outside of the casting and a bunch of clean rags to wipe it down. Use a turkey baster to suck up a little fluid if it's too full. If the cap doesn't have the fiber gasket it might be a good idea to buy or make one.
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!