Master Cylinder Bore

Brake fluid is very creepy. It loves to creep, and will move thru the tiniest of openings.

I agree with the geyser-talk. With drum brakes, a geyser is somewhat normal, because the wheel cylinders always park in the same spot. And if the self adjusters are not working 100% then eventually you are pumping more and more fluid down the line, and it all comes back when the return springs do their job.
Of course this takes more and more pedal travel to get the job done and so, one day you say what the heck?, and then the lightbulb in your head goes on,lol. Sometimes the rear shoes haven't been rubbing the drums for .......a very long time.