Introduction to brake fluid:
Brake fluid transfers the force applied by the foot on the master cylinder to the wheel brake cylinders. Brake fluid is composed of polyglycol derivatives with various additives. It is a thin fluid with a low freezing point that can move quickly through narrow lines and passages.
The image shows the brake fluid circuit of a passenger car. The system is separated diagonally:
- Left front with right rear;
- Right front with left rear.
The master cylinder pressurises the system. The brake fluid moves through the lines to the brake cylinders of the disc brakes and/or drum brakes.
The brake system of an average passenger car contains approximately 0.25 to 0.5 litres of brake fluid. This small quantity must be able to withstand pressures of 15,000 kPa at very high temperatures.
Most of the heat is dissipated via the brake discs/drums to the ambient air. A small part of it reaches the brake fluid via the brake shoes/pads and the wheel brake cylinders. Constituents of brake fluid attack paint and lacquer. If brake fluid comes into contact with the paintwork, it must be rinsed off thoroughly with water immediately.
Vapour-lock:
Vapour-lock occurs when the brake fluid starts to boil; for example when the car is stopped after prolonged braking, or drives slowly without braking. Cooling by the airstream is then limited. When brake fluid starts to boil, the boiling point is exceeded. It must therefore have a high boiling point to prevent vapour-lock from forming. With vapour in the brake system, no pressure can be built up.
Properties of brake fluid:
Brake fluid must meet certain requirements in order to ensure the reliability of the brake system. Brake fluid also offers a protective effect for the internal components; think of the cups in the master cylinder and the inside of brake lines. This paragraph lists a number of properties.
Boiling point:
The boiling point is indicated with a DOT code. DOT is an abbreviation of “Department Of Transport”.
When brake fluid boils, vapour bubbles are formed. These vapour bubbles are compressible, meaning less pressure can be built up. Water also causes corrosion in the brake system. Brake fluid must therefore have a high boiling point. Two types of boiling points are used:
- Dry boiling point: this is the boiling point of new, packaged brake fluid.
- Wet boiling point: this is the boiling point when the brake fluid has absorbed approximately 3.2% water. Brake fluid is hygroscopic (absorbs water). Water lowers the boiling point of the brake fluid. At 1% water, the boiling point is reduced by approximately 25 degrees.
| Requirements for brake fluid | DOT 3 | DOT 4 | DOT 5 | DOT 5.1 |
| Dry boiling point: | 205°c | 230°c | 260°c | 260°c |
| Wet boiling point: | 140°c | 155°c | 180°c | 180°c |
Brake fluid must be replaced every two years because over time moisture and dirt penetrate into the brake fluid. The boiling point therefore becomes lower after a number of years.
Hygroscopic:
Hygroscopic means that the brake fluid dissolves or binds water. Water causes corrosion in the brake system and vapour-lock.
Viscosity:
Brake fluid must retain the correct viscosity (thickness) at high and low temperatures.
Foaming:
Brake fluid must not be foaming.
Protection:
Brake fluid must provide protection against corrosion in, for example, the brake lines and brake hoses.
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