Brake fluid is a hydraulic liquid that transfers the force you apply through the brake pedal through the various pipes to the calipers and (if fitted) drums, which then operate to slow your vehicle.
In such a closed system, the fluid resists compression, so every bit of force you exert with your foot is transferred to the stopping end of the set-up.
What brake fluid do I need?
There are several types of fluid, grouped under two headings: those that are glycol-based and those based on silicone.
The glycol-based brake fluids include DOT3, DOT4, Super DOT4 and DOT5.1. DOT5 is a silicon-based brake fluid. Most cars use DOT4, but always check your handbook for guidance.
In general, the lower the number, the lower the fluid?s boiling point. In normal operation, brake fluid is a liquid and can?t be compressed, so the brake pads and shoes can be properly applied.
However, under hard driving, for example after a few laps around a race track or when descending a long, steep hill, the high temperatures created by the friction of the pad on the rotor can cause the brake fluid to boil and it starts turning into a gas.
This means it becomes compressible, which makes the brake pedal feel spongy, and reduces braking efficiency. This reverses once the brake fluid cools.
