Brake fluid is hygroscopic, meaning it’ll absorb water from the atmosphere, and as it does, its effectiveness diminishes. The solution, no pun intended, is to inspect the brake fluid on a regular basis, and change it according to the manufacturers schedule, which in most cases is every two years or so. Inspecting the brake fluid should take about 30 seconds. Take a look at the master cylinder reservoir sight glass (or the reservoir itself if it’s clear plastic); there should be two lines indicating the minimum and maximum fluid levels. Ideally the brake fluid should fall midway between them. If the fluid is on the low side, it’s generally an indication that the brake pads are worn, so inspect them, and check for leaks before adding fluid. In any event, never add fluid past the max line; brake fluid expands with heat, so any extra may leak past the cap vent and make a mess.