Brake fluid is a type of hydraulic fluid responsible for the activation of the car’s braking system. It is a non-compressible substance that is stored in the brake lines, putting pressure on each of the rotors located in every corner of the vehicle.

How do Brake Fluids Work?

In a hydraulic brake system, when we press the brake pedal it compresses a piston in the brake caliper. This force from the brake pedal results in a pressure inside the brake lines that causes the brake rotors to squeeze on the brake pads. This causes friction causing the wheels to stop turning and the vehicle to stop moving. This friction transforms the vehicle’s kinetic energy into heat energy which is where the brake fluid comes in.

Several different types of brake fluid exist, but we can group them all in two categories: glycol-based and silicone-based. These can also be broken down further by grade.

DOT3 and DOT4 are both glycol-based. Silicone-based fluids only work in vehicles without anti-lock brake systems.

The Difference Between DOT 3 and DOT 4

These Glycol-based brake fluids are classified by a Department of Transportation (DOT) number: 3, 4, 5.1.

These brake fluids do not have to be classified by chemical composition as there are no specific requirements by the government. This means there is no typical brake fluid formula. With that said, they do need to meet certain requirements by the government.

The specifications detail requirements for equilibrium reflux boiling point (dry and wet boiling points), kinematic viscosities, pH values, high-temperature stability, chemical stability, corrosion, water tolerance, compatibility (sludging, sedimentation, and crystallization) and resistance to oxidation.

DOT3 is the most common type of brake fluid used by daily drivers. You can expect to find that most cars and trucks use this type. Essentially it’s for vehicles that don’t use their braking system aggressively, ie don’t turn the kinetic energy into heat that DOT 3 can’t handle.

DOT4 has a higher boiling point and has found its place in racing vehicles and police cars. DOT4 has also started to gain more popularity because of increased usage of ABS and traction control.

Note: DOT4 is compatible with DOT3 but not the other way around.

Boiling Points

The basic difference between DOT3 and DOT4 is the boiling point. This is the temperature under which the fluid evaporates and also how prone to absorbing water it is. Both DOT3 and DOT4 are hygroscopic, meaning they absorb water.

Due to its lower boiling point, DOT3 is more prone to water absorption. Consequently, DOT3 boils under hard braking much easier, which makes it less suitable for activities we’ve outlined above.

There are two kinds of boiling points — dry boiling point and wet boiling point. The dried boiling point is determined by using the fluid from a brand new container. On the other hand, the wet boiling point is determined by using a fluid that has been contaminated by 3.7% water. The latter represents a real-world scenario as outlined by the DOT in their testing environments.

Remember, there is a possibility of moisture entering the system every time you remove the reservoir cap to add fluid. This will make your fluid’s quality degrade. So make sure to flush the brake system from time to time in order to remove the moisture.

Dry Boiling Point Wet Boiling Point
DOT 3 205 °C. (401 °F.) 140 °C. (284 °F.)
DOT 4 230 °C. (446 °F.) 155 °C. (311 °F.)

Chemical Structures

Again, as noted above there are no specific requirements for the chemical structure as long as they meet the requirements we outlined.

DOT3 brake fluid is usually based on diethylene glycol (DEG). Again, this isn’t a requirement but it seems it’s the most economical way for manufacturers to meet the outlined requirements. In essence, the brake fluid industry has self-regulated and determined this to be the standard.

DOT4 consists of glycol and borate ester. The borate allows for the brake fluid to handle higher temperatures. As outlined in the chart above, the dry and wet boiling points are greater.

Conclusion

In short, we can say the main difference between DOT3 and DOT4 fluids are their boiling points. They don’t vary much other than DOT4 containing borate to increase boiling temperatures.

When it comes to changing brake fluid, the best thing to do is consult the manufacturer’s specifications. The answer to this question seems to be all over the place and answers range from being based on mileage as well as time since last brake fluid change.

References

Richard Reed
Author

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