The brake fluid must transmit the pedal force applied by the driver of a car to the wheel brakes. As soon as the driver presses the brake pedal, the force from his foot is exerted on the brake servo via the pedal. The master brake cylinder converts the amplified foot force into hydraulic pressure. This hydraulic pressure is then transmitted to the wheel brakes via brake lines and brake hoses.
The brake fluid acts as a transmission medium in this process. It must meet strict requirements. For example, it must not damage rubber seals. It must protect the components of the brake system against corrosion and wear and, above all, it must be temperature-resistant. This is because some of the heat generated during braking is transferred to the brake fluid. This results in high temperatures which, in extreme cases, can cause the brake fluid to boil. However, when the brake fluid starts to boil, the brake pedal slackens and the braking power drops significantly. When brake fluid boils, vapour locks are produced that can be compressed. Brake pulses are no longer sent to the wheel brakes, the brake pedal goes all the way to the floor and that dreaded moment when nothing happens at all occurs.
It is for this reason that the boiling point of a brake fluid is so important. We speak in terms of the dry boiling point and the wet boiling point. The dry boiling point describes the property of a new brake fluid that is still in its sealed container. It is usually between 240°C and 280°C.
On account of its composition, brake fluid has hygroscopic properties. This means that it draws in moisture from its surroundings, primarily through the brake hoses. As a consequence, the water content of the brake fluid increases over time and the boiling point drops. The temperature known as the wet boiling point is reached at a water content level of 3.5%. Once this point in time has been reached, the brake fluid must be changed.
The actual boiling point of the brake fluid can be determined in a garage using a test device. This test should be carried out annually. To safeguard the function of the brake system, the quality of the brake fluid must meet the specifications defined by the vehicle manufacturer. Furthermore, the prescribed intervals for changing the brake fluid must be observed.
The viscosity of the brake fluid is also very important. It is viscosity which safeguards the function of various brake systems. In modern control systems such as ABS or ESP®, very low viscosity is a prerequisite for absolutely reliable control processes in fractions of seconds. The hydraulic units in these systems have numerous small bores and channels, some of which are smaller than the diameter of a human hair. Selecting a brake fluid with the wrong viscosity can have fatal consequences for the function of modern brake systems.
Normally when you leave your car for service it is checked by experts such as A1 Clutches, however it is recommended by manufacturers to have your brake oil changed after every two years.