[KTL]

My "new" Corvette has two rubber lids on the reservoirs for the brake master cylinder. The lids have accordions built into them that allow the fluid level to vary depending on pad wear and heat expansion of the fluid.

The fluid gets contaminated even in a "sealed" system. I recently flushed the fluid in the Corvette and it seemed like it had never been changed, it was so brown. Therefore I have little faith in the car's master cylinder, caliper flex hoses and the caliper seals. So those are next on the list of things to be fixed/replaced. I'm certain the clutch fluid is equally bad because the clutch pedal feels as mushy as the brakes.......

My 2017 Silverado has a giant reservoir with what appears to be a sizeable enough air void above the max fluid level to allow for fluid fluctuation and expansion.