Brake fluid must have a high boiling point to avoid vaporizing in the lines at high temperatures. Vaporization is a problem because vapor is highly compressible relative to liquid, and negates the hydraulic transfer of braking force - so the brakes will suddenly fail to stop the car. Boiling points are measured as "dry" and "wet" boiling points. Dry refers to fresh fluid from a new bottle. The wet boiling point refers to the fluid's boiling point after absorbing a certain amount of moisture. Most professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed/changed every 1–2 years under non-racing conditions and much more frequently in track or racing use.
Boiling points are not the only critical factors: compressibility and miscibility are other factors to consider. A fluid's quality is reflected in its DOT rating - DOT3, DOT4, and DOT5. Most commonly these ratings are used to compare boiling points but they are also graded on compressibility, and its viscosity at different temperatures. Some fluids are called racing fluids because even though they meet or exceed DOT standards they are not intended for road use. DOT-approved fluids must be colorless or amber except for DOT 5 silicone, which must be purple.