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Are DOT 3 and 4 brake fluid the same?

No, DOT 3 and DOT 4 brake fluids are not the same. While they are both types of hydraulic brake fluid used in vehicles, they have distinct differences in their chemical composition and performance characteristics.

Understanding DOT 3 and DOT 4 Brake Fluids

Brake fluids are classified by the Department of Transportation (DOT) based on their boiling points and other properties. DOT 3 and DOT 4 are the two most common types of brake fluids used in modern vehicles.

Key Differences Between DOT 3 and DOT 4

Boiling Point: DOT 4 brake fluid has a higher dry and wet boiling point compared to DOT 3. This means DOT 4 can withstand higher temperatures before boiling, making it more suitable for high-performance or heavy-duty braking systems.

Hygroscopicity: DOT 4 brake fluid is more hygroscopic, meaning it absorbs moisture from the air more readily than DOT 3. This can lead to a lower boiling point over time as the fluid becomes contaminated with water.li>
Chemical Composition: DOT 4 brake fluid contains borate esters, which provide enhanced high-temperature performance compared to the glycol ethers found in DOT 3 fluid.

Applications: DOT 4 is typically recommended for vehicles with high-performance braking systems, such as sports cars, SUVs, and trucks. DOT 3 is more commonly used in standard passenger vehicles.

It's important to always refer to your vehicle's owner's manual to determine the recommended brake fluid type and follow the manufacturer's guidelines for maintenance and replacement.

Can I mix DOT3 and DOT4 brake fluid?

Yes, you can mix DOT 3 and DOT 4 brake fluid because they are both glycol-based and compatible. However, mixing them will lower the performance characteristics of the DOT 4 fluid, specifically its higher boiling point, so it is best to only mix them in an emergency. For optimal performance and safety, always use the recommended brake fluid for your vehicle, which is typically specified in the owner's manual.
What to know before mixing

They are compatible: DOT 3 and DOT 4 are chemically compatible and won't damage your brake system's rubber components.
DOT 4 has a higher boiling point: The primary difference is that DOT 4 has a higher boiling point, which is better for high-performance braking.
Mixing lowers performance: Adding DOT 3 to DOT 4 will lower the overall boiling point of the fluid mixture, reducing its performance advantage.
Switching requires a flush: If you want to switch to DOT 4 after using DOT 3, you must completely flush the system to remove all the old fluid before adding the new fluid to take full advantage of the DOT 4's higher boiling point.

What to do

Use the correct fluid: Always check your owner's manual and use the specific type of brake fluid recommended for your car.
Don't mix DOT 5: Never mix DOT 5 fluid (which is silicone-based) with DOT 3, 4, or 5.1, as they are not compatible.
Perform a full flush: If you need to change to a higher grade of fluid, a full system flush is recommended for best results.
Top off cautiously: If you are just topping off a DOT 3 system, using DOT 4 is acceptable in an emergency, but it's best to stick with the correct fluid if possible.

Which brake fluid is better, DOT 3 or DOT 4?

You should use DOT 3 or DOT 4 brake fluid based on your vehicle's manufacturer specifications, which can be found in your owner's manual. If your car requires DOT 3, you can upgrade to DOT 4 for its higher boiling point, which is beneficial for high-temperature braking or frequent use, but do not use DOT 3 if your car calls for DOT 4.
When to use DOT 3

Daily driving: Most standard cars and trucks use DOT 3 for moderate braking conditions.
If specified: Always use DOT 3 if it is the fluid specified by your vehicle's manufacturer.

When to use DOT 4

For higher performance: DOT 4 has higher boiling points, making it better for high-performance vehicles or situations where brakes are used frequently, like in hilly terrain or towing.
If specified: Some modern vehicles, especially those with ABS systems, may specifically require DOT 4.
Upgrading from DOT 3: If your car currently uses DOT 3, you can upgrade to DOT 4 for improved performance and a higher safety margin. The two are compatible and can be mixed, though it's generally recommended to use the higher-spec fluid if you have the option.

Key differences

Boiling points: DOT 4 has higher dry and wet boiling points than DOT 3, meaning it can withstand higher temperatures before it begins to boil and lose effectiveness.
Moisture absorption: Both fluids are hygroscopic, meaning they absorb moisture over time, which lowers their boiling point. Because DOT 4 has a higher starting boiling point, it has more of a buffer before becoming a safety risk.

Important considerations

Check your manual: Always refer to your car's owner's manual for the correct brake fluid specification to ensure compatibility and prevent system damage.
Do not use lower-spec fluid: Never use a lower-spec fluid (like DOT 3) in a vehicle that requires a higher-spec fluid (like DOT 4).
Do not mix DOT 3 and DOT 5: While DOT 3 and DOT 4 are compatible, DOT 5 is silicone-based and should not be mixed with either glycol-based DOT 3 or DOT 4 fluids.

What happens if I use DOT 4 instead of DOT 3?

Using DOT 4 instead of DOT 3 is generally acceptable and can even offer better performance because DOT 4 has a higher boiling point, which is beneficial under heavy braking. However, be aware that DOT 4 can absorb moisture slightly faster, potentially requiring more frequent fluid changes.
What to know

Higher boiling point: DOT 4's higher boiling point (446∘F446 raised to the composed with power cap F446∘𝐹 or 230∘C230 raised to the composed with power cap C230∘𝐶) compared to DOT 3's (401∘F401 raised to the composed with power cap F401∘𝐹 or 205∘C205 raised to the composed with power cap C205∘𝐶) can help prevent brake fade during hard or sustained braking, such as when driving down steep hills.
Hygroscopic: Both DOT 3 and 4 are glycol-based and absorb moisture from the air over time. Since DOT 4 absorbs moisture a bit faster, it's good practice to change your brake fluid every two years to maintain optimal performance, regardless of the type used.
Compatibility: DOT 3 and DOT 4 brake fluids are fully compatible and can be mixed without causing damage to your brake system's seals or components because they are both glycol-based.
Limitations: While it's okay to use DOT 4 in a system that calls for DOT 3, you should not use DOT 3 in a system that specifically requires DOT 4, as you would lose the benefit of the higher boiling point.

What vehicles use DOT 4?

DOT 4 is used in some euro and domestic vehicles. DOT 4 Plus is used in Mercedes and Volvo. DOT 4 Low Viscosity is used in some BMW models. Finally, DOT 4 Racing usually has an added blue color.

Kevin Bennett

Company Owner

Kevin Bennet is the founder and owner of Kevin's Autos, a leading automotive service provider in Australia. With a deep commitment to customer satisfaction and years of industry expertise, Kevin uses his blog to answer the most common questions posed by his customers. From maintenance tips to troubleshooting advice, Kevin's articles are designed to empower drivers with the knowledge they need to keep their vehicles running smoothly and safely.