DOT brake fluid classification and description

Brake fluid is a special substance that fills the braking system of a car and plays an essential role in its operation. It transfers the force from pressing the brake pedal through the hydraulic drive to the braking mechanisms, due to which the vehicle is braked and stopped. Maintaining the required quantity and appropriate quality of brake fluid in the system is the key to safe driving.

Purpose and requirements for brake fluids

The main purpose of the brake fluid is to transfer power from the main brake cylinder to the brakes on the wheels.

The vehicle's braking stability is also directly related to the quality of the brake fluid. It must meet all the basic requirements for them. Additionally, you should pay attention to the manufacturer of the fluid.

Basic requirements for brake fluids:

1. High boiling point. The higher it is, the less likely is the formation of air bubbles in the liquid and, as a result, a decrease in the transmitted force.
2. Low freezing point.
3. The fluid must maintain the stability of its properties throughout its entire service life.
4. Low hygroscopicity (for glycol bases). The presence of moisture in the fluid can lead to corrosion of the brake system components. Therefore, the liquid must have such a property as minimal hygroscopicity. In other words, it should absorb moisture as little as possible. For this, corrosion inhibitors are added to it, protecting the elements of the system from the latter. This applies to glycol-based fluids.
5. Lubricating properties: to reduce wear of brake system parts.
6. No harmful effects on rubber parts (O-rings, cuffs, etc.).

Brake fluid composition

Brake fluid consists of a base and various impurities (additives). The base makes up to 98% of the composition of the liquid and is represented by polyglycol or silicone. In most cases, polyglycol is used.

Esters act as additives, which prevent the oxidation of the liquid with atmospheric oxygen and with strong heating. Also, additives protect parts from corrosion and have lubricating properties. The combination of the components of the brake fluid determines its properties.

You can only mix liquids if they consist of the same base. Otherwise, the basic performance characteristics of the substance will deteriorate, which can lead to damage to the elements of the brake system.

Classification of brake fluids

Brake fluids are classified into several types. The classification is based on the boiling point of the liquid and its kinematic viscosity according to the DOT (Department of Transportation) standards. These standards are adopted by the US Department of Transportation.

Kinematic viscosity is responsible for the fluid's ability to circulate in the brake line at extreme operating temperatures (-40 to +100 degrees Celsius).

The boiling point is responsible for preventing the formation of a vapor lock that forms at high temperatures. The latter can lead to the fact that the brake pedal does not act at the right time. The temperature indicator usually takes into account the boiling point of "dry" (without water impurities) and "wetted" liquid. The proportion of water in the "humidified" liquid is up to 4%.

There are four classes of brake fluids: DOT 3, DOT 4, DOT 5, DOT 5.1.

1. DOT 3 can withstand temperatures: 205 degrees - for a "dry" liquid and 140 degrees - for a "humid" one. These fluids are used under normal operating conditions in vehicles with drum or disc brakes.
2. DOT 4 is used on vehicles with disc brakes in urban traffic (acceleration-deceleration mode). The boiling point here will be 230 degrees - for a "dry" liquid and 155 degrees - for a "humid" one. This fluid is most common in modern cars.
3. DOT 5 is silicone based and is incompatible with other fluids. The boiling point for such a liquid will be 260 and 180 degrees, respectively. This liquid does not corrode paint or absorb water. As a rule, it does not apply to production cars. It is usually used in special vehicles operating in extreme temperatures for the braking system.
4. DOT 5.1 is used in sports cars and has the same boiling point as DOT 5.

The kinematic viscosity of all types of liquids at a temperature of +100 degrees is no more than 1,5 sq. mm / s., and at -40 - it differs. For the first type, this value will be 1500 mm ^ 2 / s, for the second - 1800 mm ^ 2 / s, for the latter - 900 mm ^ 2 / s.

As for the advantages and disadvantages of each type of liquid, the following can be distinguished:

• the lower the class, the lower the cost;
• the lower the class, the higher the hygroscopicity;
• impact on rubber parts: DOT 3 corrodes rubber parts and DOT 1 fluids are already fully compatible with them.

When choosing a brake fluid, the car owner must follow the manufacturer's instructions.

Features of operation and replacement of brake fluid

How often should the brake fluid be changed? The service life of the fluid is set by the automaker. The brake fluid must be changed on time. You should not wait until her condition is close to critical.

You can visually determine the state of a substance by its appearance. The brake fluid must be homogeneous, transparent and free of sediment. In addition, in car services, the boiling point of a liquid is assessed with special indicators.

The required period for inspecting the condition of the liquid is once a year. Polyglycolic fluid needs to be changed every two to three years, and silicone fluid - every ten to fifteen years. The latter is distinguished by its durability and chemical composition, resistant to external factors.

Conclusion

Special requirements are imposed on the quality and composition of the brake fluid, since the reliable operation of the brake system depends on it. But even high-quality brake fluid tends to deteriorate over time. Therefore, it is necessary to check and change it in time.