Never read it anywhere, but I've always wondered if one purpose of the "bobber" was to cover the exposed surface area of the brake fluid, (which is very hydroscopic, meaning it absorbs water) to minimize moisture absorption rate into the brake fluid..

and, as mentioned above, the bobber turns on the low brake fluid light

2000 4Runner,V6,Auto,31's, Sliders, Skids, On-Board Air, Air Shocks
Related Interests: Welding, Ham Radio, Road Tripping (Blue Highways), High End Video/Audio Systems