Brake Fluid Boiling Point

DOT standards set minimum dry and wet boiling points for each fluid type. Glycol-based fluids absorb water from the air over time, which lowers the boiling point. The wet boiling point shows performance after the fluid has absorbed some water. DOT 3, DOT 4 and DOT 5.1 are glycol-based and can generally be mixed if the car allows it, while silicone-based DOT 5 must never be mixed with them.

As fluid absorbs moisture, the water boils at a much lower temperature than the fluid itself.

A low boiling point can cause a long or spongy pedal after heavy braking, for example on mountain descents. Change brake fluid at the interval specified by the manufacturer.