brake fluid

Brake fluid is how a push on the pedal becomes a squeeze at the wheel. The system is a closed hydraulic circuit: the pedal moves a piston in the master cylinder, and because liquid does not compress, that force is carried through the fluid to a piston at each caliper, which presses the pads against the discs. It also multiplies force: a small piston pushing on a large one trades distance for pressure, and the fluid is the medium, not the source.

It is the one fluid on the car changed on a time schedule, not a mileage one, and the reason is water. Glycol-based fluid is hygroscopic: it absorbs moisture from the air over the years, which lowers its boiling point. A hard stop heats the caliper, that water turns to vapor, vapor compresses where liquid did not, and the pedal sinks toward the floor — fade with nothing dripping to warn anyone.

Use the grade the manufacturer calls for, keep it off paint, where it is corrosive, and never top up a leak instead of repairing it — a falling level means fluid is escaping, and the pedal finds out where at the worst moment.