The brake fluid is contained within the system assembly. When pressure is applied to the service foot brake, the master cylinder exerts a force of fluid. This force actuates the wheel cylinders to push the brake shoes against the brake drums or the pads against the discs, resulting in the braking action. This brings the brake temperatures to high levels. If the brake fluid has been contaminated by excessive usage of high heat the boiling point can greatly be reduced in a short period of time.
Air can contain varying amounts of moisture depending upon the relative humidity.
This moisture can find its way into the brake system through flexible hoses, thus contaminating the fluid. Moisture contamination directly results in reduction in the brake fluid boiling point. The frictional components in a braking system generate very significant amounts of heat, especially vehicles being operated in severe conditions like a police vehicle or service vehicle in mountainous regions. This combination of higher heat and a reduced boiling point increase the possibility of fluid vaporization or vapor lock. Unlike brake fluid, vapors are highly compressible.