DOT 3 vs. DOT 4 Brake Fluid: What’s the Difference?

Brake fluid is not all that well understood. We all know our cars need it, but that doesn’t mean we have a grasp on what it does, or what the letters and numbers for each brake fluid type means. Some people don’t know that there are different ratings for brake fluid and that they can affect how your vehicle performs. For instance, do you know the difference between DOT 3 and DOT 4 brake fluid? Most car owners don’t.

Understanding what brake fluid does, what the ratings mean, and the difference between these types of brake fluids can help you make better decisions for the long-term care and maintenance of your car’s braking system.

How Brake Fluid Works

Before we start explaining the difference between DOT 3 and DOT 4 brake fluid, you need a basic understanding of what brake fluid does. This will give you an idea of how the different ratings impact your brake system performance.

Most cars use a hydraulic braking system. When you press the brake pedal, this pressure sends brake fluid into your calipers, which press the pads into the rotors. As you apply more pressure, the pads and rotors will generate more friction and the car will come to a stop more quickly.

Without brake fluid, you won’t be able to stop. Additionally, you should change your brake fluid from time to time, because it does go bad over time. It can absorb moisture, which can prevent your brakes from working properly. Bad brake fluid can cause your brake system to malfunction, which can be deadly. Ultimately, maintaining your brake fluid is crucial for safe driving.

For those wondering why moisture is so bad for the fluid, it is simple. Fresh brake fluid has a boiling point that is a higher temperature than would be produced while braking. However, water has a much lower boiling point and can boil from the temperatures produced during braking.

Moisture which has been absorbed by the brake fluid can boil, releasing gas. With this gas in your braking system, pressing the brake will simply compress the gas, instead of pushing the brake fluid to your rotors. Essentially, boiling brake fluid may lead to you hitting the brake and the pedal simply going to the floor without slowing the vehicle. This is why regularly replacing your brake fluid is important.

What are Brake Fluid Ratings, and What do They Mean?

autozone brake fluid under hood next to master cylinder 1

The ratings are developed in regard to your brake fluid’s boiling point. Boiling points are broken down into two categories. The dry boiling point is the boiling point for fluid that hasn’t been contaminated with water or other pollutants. The wet boiling point is the boiling point for fluid that has water. Traditionally, these are rated as DOT 3 or DOT 4 brake fluid. The DOT here stands for The Department of Transportation, the group which defines the ratings.

Typically, when comparing DOT 3 vs. DOT 4, DOT 3 brake fluid has a dry boiling point of 401 degrees Fahrenheit, and a wet boiling point of 284 degrees Fahrenheit. Given the fact there is no standard formula for most brake fluids, this is an estimated boiling point, and the exact boiling points for these may vary. Typical DOT 3 fluids easily exceed these values. DOT 3 fluid typically includes 80% glycol ether base which is one of the factors that contribute to water absorption.

DOT 4 Fluids both have higher Dry Boil Points and wet boil points which are required in brake systems that produce more heat due to faster speeds, heavier towing loads, etc. DOT 4 absorbs moisture faster than DOT 3 brake fluids and should be changed more regularly. DOT 4 fluid typically has 50 to 65% glycol ether base with 20-30 % Borate Ester which helps resist the boiling of the fluid, meaning you have 3.7% moisture in your DOT 3 fluid the fluid will boil at roughly 290°F. The same amount of moisture in your DOT 4 Brake Fluid will boil at around 330°F. The DOT 4 fluid will absorb that 3.7% water amount 20% faster than a DOT 3 Fluid.

When examining DOT 3 vs. DOT 4 brake fluid, the boiling point and lifespan are the most notable distinctions between types. You’ll have to replace DOT 4 more often than DOT 3 brake fluid.

There are also DOT 5 and DOT 5.1 brake fluid ratings, however, these types of brake fluid are far less common. DOT 5 is silicone based, whereas DOT 5.1 was designed with less than 70% silicone, which allows it to be more compatible with other braking systems than DOT 5, which is incompatible with anti-lock brakes and fluid ratings 3 and 4. DOT 5.1 Brake Fluid is 20-30% Glycol Ether based 50-70% Borate Ester and is compatible with DOT 3 or DOT 4 Brake Fluid. That said, you should always use brake fluid that is the same rating that your vehicle’s manufacturer recommends in the owner’s manual. You can also check what kind of fluid your car takes at AutoZone.

DOT 3 vs DOT 4 Brake Fluid

When comparing the differences between the two, you should keep in mind that as there is no standard formula for brake fluids, and each type of brake fluid may contain different compounds or chemicals, there are only a few differences that distinguish DOT 3 vs. DOT 4 brake fluid.

The primary differences between the two include the following:

  • DOT 3 brake fluid will absorb less water than DOT 4 from the air over time, meaning you’ll need to have your fluid changed less frequently.
  • DOT 4 brake fluid has higher dry and wet boiling points, making it safer for higher temperatures.

Ultimately, the difference between DOT 3 and DOT 4 brake fluids lies in their ability to handle the heat and water absorption. When you need your brake fluid changed, you should consult your owner’s manual to see what type of brake fluid your car recommends. You should not mix different types of brake fluid, as this can cause your brakes to malfunction due to incompatibility.

If you’re uncertain what type of brake fluid you’ll need, you can reach out to us at AutoZone and one of our team members will help you determine what your vehicle requires.

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