How often and why change the brake fluid?

Brake fluid is an important part of a vehicle’s braking system. Your safety depends on its quality. If you regularly change the working fluid and do not forget to check the serviceability of the system, it will last a long time, and expensive repairs will not be taken by surprise. For what reasons and how often it is necessary to change the working fluid, we will figure it out in this article.

Varieties of brake fluid

Different types of fluid work in modern braking systems. The first formulations were based on a mixture of butyl alcohol and castor oil. At low temperatures (below 0 ° C), such a liquid became thick and partially lost its properties. Its complete replacement was required annually.

As the automotive industry has developed, more advanced brake fluids have been developed. All of them are combined in the framework of standards (so-called DOT), indicating the boiling point and the degree of viscosity depending on the environment. The most common TAs:

DOT 3 is a glycol version of the liquid, based on two substances: polyethylene glycol and polyester. This type of brake fluid is suitable for vehicles with modest dynamic performance. The operating temperature in the braking system of such vehicles should not exceed 150 ° C. It is permissible to mix the DOT 3 liquid with a similar composition of the DOT4 standard TJ.

DOT 4 is a composition based on polyethylene glycol and polyester, supplemented with special additives to prevent boiling. The most popular type of brake fluid is effective at heating the system up to 200-230 ° C, which is optimal for most modern cars. Compatible with DOT 3 fluid.

DOT 5 is a brake fluid formulated with silicone. Specially formulated for high-speed sports cars, DOT 5 fluid provides trouble-free braking performance at boiling temperatures up to 260 ° C. Due to its special composition, DOT 5 must not be mixed with glycol-based fluids. At the same time, the life of silicone-based fluid is only 1 year.

DOT 5.1 / ABS is a glycol formulation used exclusively in braking systems equipped with an ABS mechanism. Mixing a specialized brake fluid with others is undesirable.

Reason for replacement

The working cylinders in the brake system of the car communicate with the main one by means of the brake fluid. Thanks to their well-coordinated work, the movement of the master cylinder piston is transmitted to the brake cylinders, and then to the pads. Brake fluid is subject to constant heating, but due to its special composition, it does not boil, even if the temperature in the system is above 100 degrees for a long time.

The disadvantage of the composition is its hygroscopicity. Brake fluid is able to absorb moisture from the external environment. If it contains more than 3% water, the liquid will lose its effectiveness. When depressurizing, air can enter the system with vapor hazardous to the brake fluid. In other words, when air from the system and water enters, the boiling point of the brake fluid will drop sharply and at a certain moment it will begin to boil. Despite the fact that if the tightness of one circuit of the brake system is lost, the second retains the ability to stop the vehicle in time, even a partial failure of the general mechanism can cause an accident.

When boiling, the working volume of the liquid becomes larger and when you press the brake pedal, the “excess” liquid is poured into the reserve tank. After cooling, compression occurs and the previously displaced volume may not be enough to create the required pressure in the system. In such cases, the driver feels the brake pedal fall through without resistance, which is very dangerous when driving at high speeds. That is why the level of the brake fluid must be monitored and precisely known when to change the brake fluid.

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