Brake fluid is a main working substance in the brake system hydraulic drive. It transmits force from the master cylinder to wheel cylinders, due to which the brake pads are pressed against the discs, and braking is performed.

Classification of brake fluids

According to the US Department of Transportation (DOT) classification, the brake fluids used nowadays can be divided into the following types:

• DOT 3. They are used in the cars designed for use at small and medium speeds. They are mostly designated for the systems with drum brake mechanisms on both axles, or the systems having the discs on the front axle and drums – on the rear one. Products of this category meet the following requirements: their boiling temperature is not lower than 205 and 140 °C, for a clean product and the product with 3.5% water content, correspondingly; and their kinematic viscosity at 100°C temperature should not be less than 1.5 mm2/s, at −40 °C – no less than 1500 mm2/s.
• DOT 4. They are designed for the high speed cars with disc brake mechanisms, including ventilated ones, on both axles. They have the following characteristics: boiling temperature without water – no less than 230 °C, with 3.5% content of moisture – higher than 155 °C; kinematic viscosity at −40 °C and 100 °C – no higher than 1800 and no lower 1.5 mm2/s.
• DOT 5.1. These products are made for sport cars, braking system of which operates in conditions of high temperature loads. They have 260–180 °C boiling temperature, and kinematic viscosity of about 1.5–900 mm2/s at the temperature above and below zero correspondingly.
• DOT 5. They correspond to the same requirements as 5.1, but are made using a different base. As a rule, they are used in racing cars and other sportive vehicles.
• DOT 4 Class 6. They comply with stricter standards approved basing on ABS system requirements. They have boiling temperature of 250–165 °C range, and retain excellent fluidity at intense cold: their viscosity doesn’t exceed 750 mm2/s.

This classification doesn’t apply to the mineral-based products. They should meet other requirements.

Types of brake fluids by formulation

• Mineral ones. Purified petroleum fractions lie in their basis. Such products have excellent lubricating properties, securely protect the system parts from destruction, do not absorb moisture, cause no destructive effect on sealing elements and paintwork. Moreover, cost of the mineral-based brake fluids is lower than that of synthetic compounds. These products are mostly used in previous generations of cars, as they do not meet the modern standards.
• Glycolic ones. They are products made on the basis of polyglycols and their ethers. They have enhanced characteristics. But they are apt to absorb atmospheric moisture, which leads to lowering the initially high boiling temperature during use, and also causes increase of low-temperature viscosity. They are used in mostly all modern cars. They are not compatible with sealing elements of previous generation cars.
• Silicon ones. They are made on the basis of organosilicon polymers. They have stable viscosity regardless to a temperature. They do not absorb moisture, but have insufficient lubricating properties and high price. Moreover, they are apt to aeration, that is why they are not allowed for use in systems with ABS.

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