A dangerous condition exists when brake fluid boils in the braking system; it can feel like air trapped in the lines when braking. In extreme cases the driver may be able to push the brake pedal completely to the floor without slowing or stopping the vehicle. In colder climates, brake fluid with high water content becomes very thick, which can also cause slow pedal response or difficulty braking. Also, as the water content in brake fluid increases, it will become more corrosive to steel pistons and the ABS modulator.
Brake fluid with high water content has a reduced boiling point. This means that the brake fluid will boil in the wheel cylinder at a lower temperature than the braking system was designed for. When traveling down a steep grade, towing a trailer, or in the event a brake pad becomes stuck against a rotor, brake fluid temperature can increase high enough to boil if it contains enough water. Stepping on the brake pedal will now only compress the vapor instead of applying force to the pad – the result is sudden brake failure.
DOT Standard 116 requires that DOT3 brake fluid, with no moisture content, will only boil at temperatures above 400 °F (205 °C).
Scale 1 Fluid: Brake Fluid - DOT3; Unit of Measure: % Water; Range: 0 to 6; Resolution: 0.1; Precision: +/-0.5
Scale 2 Fluid: Brake Fluid - DOT3; Unit of Measure: Boiling Point °F; Range: 244 to 485; Resolution: 1; Precision: +/-5
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