DOT 3 vs DOT 4 Brake Fluid (How They Differ)

The main difference between DOT 3 and DOT 4 brake fluid is their boiling points. DOT 4 has a higher boiling point, making it more resistant to brake fade in high-temperature conditions.

When it comes time to replace your brake fluid, understanding the difference between DOT 3 and DOT 4 brake fluid will help make sure you choose the right fluid for your vehicle.

This article will explain the key distinctions between DOT 3 and DOT 4 brake fluid types in terms of composition, dry and wet boiling points, and recommended uses. We’ll also tell you what happens if you accidentally mixed in the wrong brake fluid.

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Differences Between DOT 3 and DOT 4 Brake Fluid

Most car manufacturers use either DOT 3 or DOT 4 brake fluid despite newer DOT 5 and 5.1 fluids available on store shelves as well. These two brake fluids have some key differences:

#1 – Boiling Point

One of the most important distinctions between DOT 3 and DOT 4 brake fluid is their boiling point. The boiling point is the temperature at which the fluid starts to vaporize and turn into a gas. It’s measured in two ways.

Dry Boiling Point

The dry boiling point is the temperature at which a fluid boils when there is no outside moisture present. In other words, this is the brake fluid in the container it came in.

For DOT 3 fluid, the dry boiling point is typically 401°F (205°C). DOT 4 fluid has a higher dry boiling point of 446°F (230°C).

Wet Boiling Point

The wet boiling point refers to the temperature at which a fluid starts to boil after it has absorbed some moisture from the air. Even with a tightly sealed brake fluid reservoir and brake system in general, over time, brake fluid can absorb small amounts of water from the atmosphere.

For DOT 3 fluid, the wet boiling point is 284°F (140°C) while DOT 4 fluid has a higher wet boiling point of 311°F (155°C).

Why does this matter? Brakes can generate extreme amounts of heat from friction when stopping your vehicle. If the brake fluid reaches its boiling point from this heat, it will form bubbles and pockets of vapor.

This vapor is compressible, unlike liquid brake fluid, which causes your brakes to feel soft and spongy. This dangerous condition is known as brake fade.

The higher boiling point of DOT 4 brake fluid makes it much more resistant to brake fade. Hard braking when towing or descending long hills can bring DOT 3 fluid close to its boiling point. DOT 4 fluid has a safety margin before vaporization occurs.

#2 – Chemical Components

The major chemical components that distinguish DOT 3 and DOT 4 brake fluids are glycol and glycol ether compounds.

DOT 3 brake fluid consists of triethylene glycol and triethylene glycol monoethyl ether. The glycol gives the fluid a relatively low boiling point while the glycol ether helps minimize water absorption.

DOT 4 fluid contains polyethylene glycol along with diethylene glycol. Polyethylene glycol has a higher boiling point, providing resistance to brake fade at high temperatures. The diethylene glycol still absorbs more water than DOT 3 fluid, but has a higher boiling point.

The different glycol bases are primarily responsible for the variations in boiling point thresholds between the two brake fluid types. Glycol ethers in both impact hygroscopic properties (how readily they absorb moisture from the air over time). Too much water content in brake fluid leads to corrosion in brake system components.

While the chemical makeup may sound complex, the takeaway is that DOT 3 and DOT 4 have different glycol and glycol ether formulations to achieve different brake fluid performance. DOT 4’s chemical composition gives it the clear advantage for demanding braking systems and severe duty in high heat.

#3 – Recommended Applications

DOT 3 brake fluid is suitable for use in most standard passenger vehicles, light trucks, and vans. Unless specified otherwise by the manufacturer, DOT 3 fluid meets the needs of everyday driving and normal braking demands.

DOT 4 fluid is required for high-performance vehicles with advanced braking systems. Sports cars, racing vehicles, luxury models, police vehicles, and ambulances often necessitate DOT 4 for fade resistance during repeated hard stops.

DOT 4 may also be specified for heavier vehicles like commercial trucks and buses that have air brakes and hydraulic systems. Heavy duty towing and hauling generates significant heat, making DOT 4 the ideal choice.

While DOT 3 fluid can be used in any vehicle specifying DOT 4, the reverse is not true. Using DOT 3 fluid in a vehicle designed for DOT 4 is unsafe and will lead to diminished braking ability. Always check your owner’s manual to see which brake fluid is recommended by the manufacturer.

Accidentally Mixed DOT 3 and DOT 4?

It’s understandable to accidentally mix up DOT 3 and DOT 4 brake fluid. The containers look similar and it’s easy to grab the wrong one (or make the simple mistake of buying the wrong kind). But what should you do if you’ve made this mistake?

First, don’t panic. The mixed fluid will still allow you to safely operate your brakes, at least in the short term. However, the boiling point and moisture absorption properties are now compromised if you added DOT 3 fluid to a vehicle that is supposed to take DOT 4.

The first issue is boiling point. When mixing DOT 3 and DOT 4 fluid, the boiling point of the resulting mixture will be somewhere between the boiling ranges of the individual fluids. This may still provide adequate performance, but optimal fade resistance will be reduced.

Another issue is the increased water absorption that comes from mixing in DOT 3 fluid. DOT 4 resists moisture uptake better than DOT 3. Adding any amount of DOT 3 brake fluid will increase the hygroscopic properties of the fluid.

If some DOT 3 fluid must be added to a DOT 4 system in an emergency, it should be flushed and replaced with DOT 4 as soon as possible after.

Pros and Cons of Each Type

DOT 3 Brake Fluid

Pros:

Widely available and affordable
Meets minimum standards for most passenger vehicles
Performs adequately in normal braking conditions

Cons:

Lower boiling point than DOT 4
More prone to moisture absorption over time
Not suitable for high performance or heavy duty vehicles

DOT 4 Brake Fluid

Pros:

Higher boiling point resists brake fade at high temps
Less hygroscopic so absorbs less water
Meets the demands of high performance and heavy duty vehicles

Cons:

More expensive than DOT 3
Not always easy to find at stores
Unnecessary for a basic transportation car or light truck

In short, DOT 3 fluid is economical and gets the job done for everyday driving, while DOT 4 provides superior performance for advanced braking systems. But the determination of what fluid to use should be based on what the vehicle manufacturer specifies (check the owner’s manual).

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Mark Stevens

Owner at Car Treatments

Mark is a veteran auto technician who has honed his skills with over 19 years of service working at various independent shops. While Fords and Mazdas are his specialty, he enjoys working on practically any vehicle (except a Nissan 300ZX).

He’s known for his attention to detail and problem-solving skills and enjoys coming up with out-of-the box solutions when the situation requires it.

When he's not troubleshooting customers' cars, you'll most likely find him working on his current project ('96 Bronco) or cooking Italian food for his family.

DOT 3 vs DOT 4 Brake Fluid (What’s the Difference?)