Modern braking systems use hydraulics to transfer the force from the brake pedal to the brake calipers. Brake fluid is special hydraulic fluid that can withstand the high temperatures produced by the brakes. Brakes turn the kinetic energy of your vehicle’s motion into heat.
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.
See Also: 5 Symptoms of a Brake Fluid Leak
Boiling Points
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 401°F (205°C) and its wet boiling point is 284°F (140°C).
- The dry boiling point of DOT 4 fluid is 446°F (230°C) and its wet boiling point is 311°F (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.
DOT 3 and DOT 4 brake fluids are both glycol-ether based. The difference in their boiling point comes from borate compounds that raise the boiling point of the DOT 4 fluid, compared to DOT 3.
In some cases, DOT 4 may be slightly less viscous than DOT 3 as well. This is beneficial for ABS systems that actuate valves inside the ABS module very rapidly.
Many passenger vehicles call for DOT 3. This is because you typically don’t brake as aggressively or as frequently on the street as police cars or cars on a race track would.
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.
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 race cars, police cars, and vehicles that tow heavy loads 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.
If your vehicle is compatible with DOT 4 brake fluid and you’re currently using DOT 3, you may consider changing it out at your next brake fluid flush.
What Can Happen If You Use the Wrong Brake Fluid?
Using the wrong brake fluid can negatively affect your braking performance in a very serious way. Silicone and glycol based brake fluids are incompatible. Check your owner’s manual to see which type of brake fluid is recommended for your specific vehicle.
Can DOT 3 Fluid Be Used Instead of DOT 4?
No. DOT 3 has a lower boiling point than DOT 4 and should not be used where DOT 4 is called for.
Can DOT 4 Fluid Be Used Instead of DOT 3?
Yes. DOT 4 is a direct replacement for DOT 3 and can operate safely at a higher temperature range. Sometimes DOT 4 fluid needs to be serviced on a shorter interval because it absorbs water more quickly.
Can DOT 5.1 Be Used Instead of DOT 3 or DOT 4?
Yes. DOT 5.1 is glycol based like DOT 3 and DOT 4 but stays a liquid at higher temperatures. The trade off is that DOT 5.1 may absorb water much faster than DOT 3 or DOT 4 fluids, which means it needs to be changed more often.
Can DOT 5 Be Used Instead of DOT 3 or DOT 4?
No. DOT 5 is silicone based and completely incompatible with other types of brake fluids. The naming conventions are very confusing, because DOT 5.1 is compatible with DOT 3 and DOT 4.
Can Brake Fluid Be Reused?
No, it’s highly recommended that you do not reuse brake fluid. Brake fluid absorbs water when it is exposed to air. Water lowers the boiling point of your brake fluid, making it highly unsafe to reuse old fluid.
Conclusion
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.
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DOT 4 is more corrosive than DOT 3. Putting DOT 4 in a DOT 3 system can damage all the rubber components.
This is simply not true. Do you have a source for this? DOT 3 and DOT 4 are completely compatible with each other.
It won’t happen.Dot 3, Dot 4 and Dot 5.1 can be mixed,it means they are almost the same.Dot 4 and 5.1 use mix of glycol and borate to reach higher boiling point.That’s all.Dot 3 is ok for normal use,dot 4 is better,and dot 5.1 is the best if you live in really cold region due to it’s low viscosity.
Kevin and Van Nelle are correct: it is safe to mix DOT 3, DOT 4, and DOT 5.1. DOT 5 however is different from DOT 5.1 and is not safe to mix with the other three. Clear as mud, right?
Personally, I try not to mix fluid ratings on my own vehicles for one reason only: the mixture would have a lower boiling point than using the higher rated fluid exclusively. If my car specifies DOT 3 in the owner’s manual and I decide to use DOT 4, it’s only because I have a use case that requires a higher brake heat resistance than the car was originally intended for.
For instance, I use DOT 4 on my track car even though the manual calls for DOT 3 since DOT 4 is more resistant to brake fade. I almost had to learn this lesson the hard way; excessive heat in the brake fluid will cause the fluid to boil, which increases your stopping distance tremendously as the brakes fade.
If your vehicle calls for DOT 4 or DOT 5.1 in the manual, I would not mix a fluid with a lower boiling point as this may lower the boiling point below the manufacturer’s standard for your vehicle.