Brake fluid is used in the circuits of modern hydraulically operated braking system. The fluid used must have a low freezing point, a high boiling point, be non corrosive to brake system components, have lubricating properties and a low viscosity.
Brake fluid is hydroscopic, which mean that it absorbs water from the air, which dramatically affects the properties of the fluid. This is because the water that builds up in the system has a much lower boiling point than the brake fluid, and on occasion where the brakes are used heavily, on long hill decent for example, the system will get very hot and the water will boil and turn into gas. The gas that now exists in the system is easily compressed and the driver will experience a spongy brake pedal or lose braking completely.
For safety reason manufacturers recommend that the brake fluid must be replaced every two years.
Brake fluid that is in good condition is typically a light yellow opaque liquid that becomes darker as it ages and will eventually turn a dirty black colour. Initially we carry out a visual inspection of the brake fluid reservoir, followed by a resistance test using an electronic probe. The resistance test determines the moisture content of the fluid by checking its resistance to electric current. If the moisture content in the fluid is excessive the resistance will be reduced and the tester will confirm that the brake fluid needs to be replaced.
When the brake fluid is replaced it is important to bleed the brake system, to remove any air from the hydraulic circuit. Each calliper, or wheel cylinder, is fitted with a release valve commonly know as a bleed nipple. These are opened in turn while the brake fluid is pumped around the system, allowing any air in the system to be exhausted from the open nipple. If any air remains in the system the brake pedal will feel spongy.