The fluid needs to be checked often because of the ability it has to absorb moisture. Excess moisture and alien liquid deteriorates the brake fluid by corroding it. Excess moisture can come from the atmosphere. The liquid that gets drawn into the fluid will also affect the boiling point which is vital for proper braking. Water boils at 100 degrees Celsius whereas clean fluid boils at 230 degrees Celsius. Vaporisation needs to be avoided in the hydraulic system as it is compressible therefore affecting the car’s ability to stop.
At times, the fluid can also contain copper. Copper is likely to release patina, which is a corrosive substance that eats away at metal. Mechanics can test if there is copper in the pipes by means of an electronic tester.
Brake fluid must have a low viscosity to flow quickly through the hydraulic system as the driver applies the pedal. In other words, it has to be thin enough to travel quickly throughout the hydraulic system as opposed to oil which is too thick. If it gets thick, the driver will notice this because of the delay in reaction time of the braking system. The colour of the liquid can also determine if you need to change the fluid. If it is darker in colour then it needs to be replaced immediately as there is a high level of contamination.
Brake fluid protects the linings of the metallic parts as well as the valves, callipers and cylinders. It also protects the metallic components from corroding. It needs to be replaced at least once every couple of years. Replacing it requires the mechanic to bleed the system. Bleeding it involves the pipe to be removed from the rest of the system. The brake pedal will need to be constantly applied to get all that liquid out.