What is the difference between DOT 3 and DOT 4?
DOT 3 fluids are used in most vehicles and intended for operation in unloaded brake systems of vehicles with disk and drum actuators. The essential advantage of these fluids is their low price. However, brake fluids of DOT 3 class are rather hygroscopic, i. e. they absorb water quickly enough. After opening a can with DOT 3, the fluid should be used as soon as possible. In the process of operation, brake fluids often absorb moisture; therefore, it is recommended to change them at least once in two years. Since DOT 3 absorbs water, any absorbed water can cause corrosion and decrease of the brake fluid boiling temperature. To avoid corrosion processes in braking main lines and cylinders, special additives are added to these fluids, which also prevent from rubber damage.
The next class is DOT 4. It combines fluids with improved performance and are intended for operation in vehicles with disk and disk ventilated brakes. In addition, DOT 4 has a higher boiling temperature than DOT 3; this is better for heavy loads, such as races, autocrosses or frequent use of brakes on mountain roads.
Temperature and viscosity indicators of DOT 3 and DOT 4 brake fluids remain stable and operable within a wide range