DOT 3, 4 and 5.1 are glycol based and consequently hygroscopic, which means they absorb moisture from the atmosphere. These fluids gradually start to absorb moisture from the moment they are put in the hydraulic brake system or exposed to the air. The fluid attracts moisture through microscopic pores in rubber hoses, past seals, due to condensation in the system and exposure to air.
As water enters the system, instead of pooling in low spots (such as the caliper), due to its weight in comparison with brake fluid, it is dispersed throughout the brake fluid. This keeps the boiling point of the entire fluid high rather than having pools of water in the system which will boil much sooner than the rest of the brake fluid.
The downside of this water absorption is that – over time – moisture greatly reduces the boiling point of the fluid.
When the boiling point of a brake fluid is severely reduced due to water absorption, the brake fluid might start to boil, especially under prolonged, hard or continuous braking (mountainous areas, heavy loads) and in hot weather. Boiling will create gas bubbles in the system. As gas is far more compressible than liquid, the driver will experience no pressure at all on the brake pedal, resulting in an extremely dangerous total loss of brake power and brake failure: vapor lock. This situation should be avoided at all times.
As a fluid that transmits force, a key property is that it is incompressible. Since it is exposed to very high temperatures during braking, brake fluid should have a very high boiling point so that it remains incompressible even during extreme braking.
Corrosion and rubber seal compatibility
Brake fluid must provide excellent corrosion protection and rubber seal compatibility.