How Brake Fluid Works
When you step on your brake pedal, it forces the small piston(s) inside your brake caliper to compress, squeezing your brake rotors and essentially slowing down your car. Although this is the simplest way to explain the concept, it doesn't factor in some of the details that allow this to happen. Because the brake pedal doesn't just magically connect to all four brake rotors, it relies on the use of brake lines to divert your single force acting on the brake pedal onto the four corners of the car. These lines are filled with a hydraulic fluid we call brake fluid.
Brake fluid works well because it's a non-compressible,
hygroscopic fluid which allows all the energy that your foot releases onto your
brake pedal to be converted into braking pressure that squeezes your brake rotors and
slows them down. So why is brake fluid so vital, yet often overlooked when shopping for brake rotors?
When you apply your brakes it causes your brake pistons to compress and clamp the brake rotors. This friction creates heat, lots of it. This means your brake fluid needs to retain its incompressible nature without boiling or evaporating due to the temperature.
What happens when your brake fluid boils or overheats? It creates air bubbles in the system and air is compressible opposed to brake fluid. So in turn, when you press down on the brake pedal, instead of clamping the rotors and slowing down the car, the brake fluid compresses the air and the brakes are not doing their job. This also translates to the driver as a mushy or unresponsive feeling brake pedal.