The braking system of a motor vehicle contains braking fluid to keep its brake components functional. Like every mechanical system of a vehicle, the braking system needs a lubricating fluid because its moving components tend to get rather hot as they’re in use.
If there is no brake fluid or if it is in poor shape, then the braking system will not function smoothly as you’re driving.
There are four types of braking fluids which are separated based on their official designation: DOT 3, DOT 4, DOT 5, and DOT 5.1. This designation represents the specific chemical mixture in which the braking fluid is comprised of.
As long as the braking fluid complies with the Department of Transportation’s minimum standards, then it is acceptable to use in the braking system of a vehicle. Out of the four types of braking fluid, the most popular ones are DOT 3 and DOT 4.
When trying to understand the difference between DOT 3 and DOT 4, you should pay attention to their two different boiling points.
There is the dry boiling point which is when you boil fresh braking fluid that has come from a new container. The other type is the wet boiling point. This refers to boiling brake fluid that has already absorbed water.
- The dry boiling point of DOT 3 fluid is 205°C and its wet boiling point is 140°C.
- The dry boiling point of DOT 4 fluid is 230°C and its wet boiling point is 155°C.
As you can see, the DOT 4 has a higher boiling point for both dry and wet points. This means it can withstand more heat before it starts to break down making it better for high performance vehicles or when braking under extreme conditions.
DOT 3 Brake Fluid
DOT 3 is considered to be the most standard braking fluid for any regular economy class car. Its chemical mixture can handle a fair amount of heat but not as much heat as a DOT 4.
The reason its boiling point is lower than a DOT 4’s boiling point is that DOT 3 brake fluid absorbs atmospheric water. This causes the performance of the fluid to be reduced along with the boiling point.
For normal passenger vehicles, this would not matter all that much because you don’t brake aggressively or as frequently as police cars or racing cars would. So, a DOT 3 will suit you fine.
DOT 4 Brake Fluid
DOT 4 is recognized as a braking fluid that is more reliable and can withstand more heat than a DOT 3. It is also known for being corrosion resistant.
Because it can handle more heat, it becomes more reliable when the brake system components have been used frequently within a short amount of time.
It can even handle a situation where other types of braking fluids were used improperly with the vehicle. That is why aggressively used vehicles, like racing cars and police cars, are known for using DOT 4 brake fluid.
DOT 4 brake fluid acts faster under a dry boiling point than a wet boiling point. The DOT 3 is able to perform well in both a dry boiling point and a wet boiling point. Overall, the DOT 4 is a very complex braking fluid which is useful for sustaining the health of your braking system.
You might wonder why DOT 3 brake fluid has not been replaced everywhere if DOT 4 is so much better. That is a good question because there is very little cost difference between these two braking fluids.
What it all comes to is the regulatory approval of DOT 3. There are more vehicles approved for DOT 3 brake fluid usage than there are for DOT 4. But this will likely change as tougher regulations become forced regarding the safety and functionality of braking systems.