Brake Fluid

Brake fluid transmits pedal force to the calipers and wheel cylinders, and because it is nearly incompressible it does so instantly. That only holds true while the fluid is in good condition.

Glycol-based DOT 3 and DOT 4 fluids are hygroscopic: they absorb moisture from the atmosphere through seals, hoses and the reservoir cap over time. As water content rises, the wet boiling point falls, and under hard or sustained braking the fluid can vaporise in the caliper. Vapour compresses, the pedal goes long and soft, and braking effort drops away. This is why manufacturers specify a fluid change every two years or so regardless of distance travelled.

Use the DOT specification stated for the vehicle, never mix in mineral oil or a silicone fluid where glycol is specified, keep containers sealed, and wipe spills off paintwork immediately.