DOT 3 vs DOT 4 Brake Fluid: What Is The Difference And Can You Mix Them?

What exactly is the difference between DOT 3 brake fluid and DOT 4 brake fluid? Can you mix DOT 3 and DOT 4? Here's a guide for you.

The brake system of a car is a hydraulic system, which means that it is dependent on a fluid – brake fluid.

But that’s not all. There are different types of brake fluid that have been categorized by the department of transportation (DOT).

This article will touch upon the differences between the different DOT classifications, with an emphasis on the difference between DOT 3 and DOT 4.

The greatest difference is in the brake fluid’s dry and wet boiling point.

However, that comes with further implications that one may not think about initially. Do they affect the performance of the brakes? Can you mix DOT 3 and DOT 4? Is the maintenance the same?

We will go through each of these questions and differences demonstrating how they affect your car’s brake system.

Stay with us and read all about brake fluids down below!

Table of ContentsShow

What Is The DOT Classification?

The U.S. Department of Transportation (DOT) has classified different types of brake fluid to make it easier and safer for consumers to use the correct brake fluid for the use of their car.

There are multiple DOT classifications ranging from DOT 1 to DOT 5.1. DOT 1 and DOT 2 are not made for modern cars.

There are other classifications for brake fluid as well, such as the International Standards Organization’s (ISO) classification and the Society of Automotive Engineers (SAE).

The Difference Between DOT 3 And DOT 4

All the DOT categories differ in boiling temperature. There are two types of boiling points, namely dry boiling point and wet boiling point.

  • DOT 3 has a dry boiling point of 401°F (205°C) and a wet boiling point of 284°F (140°C).
  • DOT 4 has a dry boiling point of 446°F (230°C) and a wet boiling point of 311°F (155°C).

What Do Different Boiling Points Mean?

Boiling Point (Temperature)

When the brakes are in use, they get hot, and so does the brake fluid.

If the brake fluid starts boiling, the brake performance will be compromised and in worst-case scenarios will be unable to stop your car.

As you may have noticed, there are two types of boiling points, a dry boiling point, and a wet boiling point.

The dry boiling point is equivalent to the boiling point of the brake fluid straight from an opened container, such as when you bought it.

When the brake fluid is in the system and operating it will get contaminated with water through micropores in the system.

The wet boiling point is defined as the boiling point when 3.7% of water has been absorbed and is now part of the whole system.

As water has a lower boiling point than what the brake fluid has initially, the more water gets mixed with the brake fluid, the lower boiling point will the brake fluid have.

It is estimated that the brake fluid will have a boiling point equal to the wet boiling point after two years of use. It is therefore important to change brake fluid regularly.

Other Parameters To Consider

The boiling point of the brake fluid is due to its composition, which not only alters the boiling point but also other characteristics.

Otherwise, it would make sense to say “DOT 4 is better than DOT 3” as the boiling point is higher in DOT 4.

The reality is that there are some advantages of DOT 3. Namely, due to its composition, DOT 3 is less hygroscopic, meaning it will attract less moisture and water from the surroundings.

This means that even though DOT 3 has a lower boiling point, it will hold that interval longer than DOT 4, assuming normal driving conditions.

Should I Choose DOT 3 Or DOT 4?

Brake Fluid

After knowing all about DOT 3 and DOT 4 you might wonder which of the two you should use for your car?

Generally, the best thing would be to follow the car manufacturer’s recommendation, however, there are some instances where one is better than the other.

DOT 3 brake fluid lasts longer as it does not attract water as much as DOT 4, as previously explained.

On the other hand, the more you use the brakes and the harder you brake, the greater the amount of heat is produced. As the DOT 4 has a higher boiling point, it would be the better choice.

If you drive in a hilly area where you brake a lot when going down or take your car to the track from time to time, DOT 4 is the better option, but be sure to change the fluid regularly.

Both DOT 3 and DOT 4 brake fluid should be changed regularly, and more often if the brakes have been used more than “normal driving”.

Can You Mix DOT 3 And DOT 4?

Brake Fluid Compatibility Chart

As both DOT 3 and DOT 4 brake fluids are glycol-based, they can be mixed without a problem but keep in mind that the boiling points will change.

Make sure to not mix DOT 3 and/or DOT 4 with DOT 5 brake fluid as the latter is silicone-based.

Summary

The difference between DOT 3 and DOT 4 brake fluid lies in the boiling point of the respective brake fluid.

There are two types of boiling points, wet and dry boiling points. Dry boiling point refers to the fluid in an unsealed container, while wet boiling point refers to when 3.7% of the water has been absorbed.

The boiling point of brake fluid is important as it can get very hot during braking. If it boils the brakes will become ineffective.

Both DOT 3 and DOT 4 brake fluids are viable choices, and they should be regularly changed, every 1-2 years depending on your driving habits.

Both abovementioned brake fluids attract moisture which lowers the boiling point.

Which brake fluid you choose does not really matter unless your car manufacturer has specified otherwise. But there are some specific circumstances where one is better than the other.

Either way, mixing DOT 3 and DOT 4 brake fluid is perfectly viable as they are both glycol-based. Just make sure not to mix it with DOT 5 brake fluid.

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Bajro Muric

Bajro is a big car enthusiast with a love for writing and teaching. He writes about anything regarding cars - from common problems and fixes to racing.