Brake fluid

The parts of the brake system are the most important parts in terms of safety on roads. As in the case of the brake disk and the brake lining, the brake fluid is part of the brake system. It is one of the main work-fluid of the hydraulic drive, which directly influence the braking effect. Vehicle owners often forget to change the brake fluid. The braking action of the vehicle tends to deteriorate if the change is neglected. The brake fluid should be changed after each 40-80 thousand km. However, besides the milage, the time elapsed from the last change shall also be considered. It is a general rule that the brake fluid has to be checked and replaced as necessary after two years even if at lower milage.
For determining the change period of the brake fluid an important factor to be considered is the hygroscopic nature of the fluid. It is because the brake fluid is capable of absorbing humidity from the air. This property of the brake fluid leads to lowering the boiling point, and a mere year is enough to have a boiling point decreased by 80 °C.
When the DOT number and the boiling point increases, then the ability to absorb humidity increases too, and for instance a fluid with DOT number 5.1 has to be changed more frequently. Take care not to mix glycol and silicone-based brake fluids, because they are not compatible.
The brake fluids are sorted in DOT categories by the
US Department of Transportation, on the basis of which nowadays they belong to the following groups:

DOT 3

Use: vehicles traveling with low medium velocity

Boiling point among dry conditions: 205 °C

Boiling point in case of 3.5% water content: 140 °C

Viscosity: At a temperature of 100°C it must be below 1.5 mm²/s, while at -40 °C it must not be below 1500 mm²/s.

DOT 4

Use: high velocity vehicles

Boiling point among dry conditions: 230 °C

Boiling point in case of 3.5% water content: 155 °C

Viscosity: kinetic viscosity at -40 °C and at 100 °C is not over 1800 and not less than 1.5 mm²/s.

DOT 5.1

Use: sports cars

Boiling point among dry conditions: 260 °C

Boiling point in case of 3.5% water content: 180 °C

Viscosity: kinetic viscosity is around 1.5–900 mm²/s above zero, and below zero degree.

DOT 5

Use: racing cars and sports cars

Boiling point among dry conditions: 250 °C

Boiling point in case of 3.5% water content: 180 °C

Viscosity: kinetic viscosity is around 1.5–900 mm²/s above zero, and below zero degree.

DOT 4 6. class

Boiling point among dry conditions: 250 °C

Boiling point in case of 3.5% water content: 165 °C

Viscosity: their viscosity is not over 750 mm²/s value

This sorting is not applicable for products made from mineral base oil. Other requirements are to be complied with by these products.

Types of brake fluids:
Mineral fluids. Used primarily in the earlier generations of vehicles.  Have excellent lubrication characteristics, protect the system elements from damage, do not absorb humidity, have no destructing influence on sealing elements, while their prices are lower. No longer complies with modern standards.
Glycol liquids: In modern vehicle such fluid is used primarily. They are capable of absorbing air humidity, which leads to the reduction of boiling point during the use, and it can cause also the increase of low-temperature viscosity.
Silicone liquids: The viscosity is stable, it does not depend on the temperature. They do not absorb humidity, but their lubrication features are not good either. They are expensive, and are liable to get mixed with air, therefore they cannot be used in systems having ABS.