Brake fluid DOT 3 vs DOT 4: what is the difference and can they be mixed?

Brake fluid DOT 3: The brake system of a car is a hydraulic system, which means that it depends on a fluid: brake fluid.

But that is not all. The Department of Transportation (DOT) has classified different types of brake fluid.

This guide will address the differences between the different DOT classifications, emphasizing the difference between DOT 3 and DOT 4.

The biggest difference is in the dry and wet boiling points of the brake fluid.

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

We’ll go through each of these questions and differences, demonstrating how they affect your car’s braking system.

Stay with us and read all about brake fluids below!

What is the DOT classification?

The US 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 their cars.

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 Organization for Standardization (ISO) and Society of Automotive Engineers (SAE) classification.

The difference between DOT 3 and DOT 4

All DOT categories differ in boiling point. 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 the different boiling points mean?

When the brakes are in use, they get hot, just like brake fluid.

If your brake fluid starts to boil, your brake performance will be compromised, and in the worst case, you won’t be able 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 brake fluid straight from an open container, just like when you bought it.

When the brake fluid is in the system and running, it will become contaminated with water through micropores in the system.

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

Since water has a lower boiling point than brake fluid initially does, the more water mixed with the brake fluid, the lower the boiling point the brake fluid will have.

Brake fluid is estimated to have a boiling point equal to the wet boiling point after two years of use. It is important to change the brake fluid periodically.

Other Parameters to Consider

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

Otherwise, it would make sense to say: “DOT 4 is better than DOT 3”, since 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, which means it will attract less moisture and water from the environment.

This means that although DOT 3 has a lower boiling point, it will maintain that range longer than DOT 4, assuming normal driving conditions.

Should I choose DOT 3 or DOT 4?

After knowing all about DOT 3 and DOT 4, you may be wondering which one you should use for your car.

In general, it would be best to follow the car manufacturer’s recommendations. However, there are some cases where one is better than the other.

As explained above, DOT 3 brake fluid lasts longer since it doesn’t attract as much water as DOT 4.

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

If you drive in a hilly area where you do a lot of braking going downhill or take your car to the track occasionally, DOT 4 is the best choice, but be sure to change your fluid regularly.

Both DOT 3 and DOT 4 brake fluid should be changed regularly and more frequently if the brakes have been used more than normal.

Can you mix DOT 3 and DOT 4?

As DOT 3 and DOT 4 brake fluids are glycol based, they can be mixed without problem, but be aware that boiling points will change.

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

How much does it cost to change brake fluid?

Brake and clutch fluid is around £10-15 per litre, and you should need between 1-2 liters for your car depending on the model.

A brake fluid change is a simple job and should take less than an hour. However, if you get the work done at a main dealer you could end up paying around £150 for labour, so it makes perfect sense to do this work yourself, with the help of a Haynes Manual.

How Brake Fluid Works

Unless you drive a very, very old vehicle, your car will have hydraulic brakes. And that’s a good thing, because hydraulic brake systems are common and also the most effective. However, they are only good if they are properly maintained.

Yes, there is the issue of discs and pads, but don’t forget about the fluid. Without this, your brakes won’t do anything. When you press the pedal, you act on the fluid, which in turn acts on the brakes.

The problem with brake fluid is that it is hygroscopic. That means it likes to absorb moisture. The more moisture that becomes part of the brake fluid, the lower the boiling point of the fluid will be. So logic would dictate that the longer you leave it on, the less effective it becomes.

Can brake fluid go bad in any other way?

If there is a failure in your brake system at any point and the fluid gets too hot, you can boil it. The lower the boiling point of the liquid, the easier it will boil. If the liquid boils, many air bubbles form, allowing the liquid to easily compress. If the fluid can be compressed, it can no longer act on the brakes when the pedal is depressed. The pedal becomes ‘soft’ and the brakes no longer work.

This can happen after a long period of braking, such as going down a long, steep hill. In fact, this can also happen when the car is being driven with gusto (on a race track, for example), where the brake fluid can overheat and cause something called brake fade, where the hydraulic action of the fluid is less effective. .

Take the brake master cylinder cap and smell the fluid, if it smells burnt or is an unusually dark color, it’s time to change it.

Your Haynes manual will guide you through the process of changing your brakes and will also cover what brake fluid your car needs. While some fluids are technically interchangeable, it’s always best to make sure you’re filling your brake system with the correct material.

How often should I change my brake fluid?

Most vehicle manufacturers recommend changing brake fluid every 2 years, regardless of mileage. The fluid may be good for longer, but why take the chance?

Even if the fluid looks fine, you still need to change it. You won’t see moisture in the fluid, it happens on a microscopic level. The best and most definitive way to know is to consult your trusty Haynes Manual.

One thing to consider about brake fluid is that even if you’re not changing it, you should still check it regularly. At the very least, you should do a cursory inspection every few weeks.

How to Change Brake Fluid

Every car will be slightly different or subject to its own intricacies. However, the general procedure remains the same, which we will now see. Before you do, there is a caveat: find a second person to help you. There are one-man bleeder kits out there, but they are rarely as effective as having a friend push the pedal for you. 

1) Drain Old Brake Fluid

Remove the cap from the brake master cylinder and using a syringe or something similar, remove as much of the old brake fluid as possible. Put it in a suitable container. Once you’ve removed as much as you can, it’s time to hit the brakes. The best way to approach this is with the car on axle stands, with all four wheels removed. 

2) Add new brake fluid

. Now you need to get a lint-free cloth and wipe away any debris that may be in the tank. Once this is done, you need to refill the master cylinder with fresh brake fluid, up to the MAX line. Use only fresh fluid from a sealed container. If the container is open, the liquid will absorb moisture from the air. What brake fluid do you need for your car? Check your Haynes manual or car manual to make sure you are using the correct specification fluid. 

3) Rinse the calipers/wheel cylinders

Each brake caliper/wheel cylinder has a bleeder nipple at the rear. You’ll need to loosen this up to let the old fluid out, but before you open it up with your keys, spray it with a bit of brake cleaner and then penetrating oil to free it up. It will be full of dirt from the road. Attach a small piece of rubber hose to the nipple, then submerge the end of the hose into the container that you put the old brake fluid from the master cylinder into. 

4) Get Help

Stick a small block under the brake pedal (this stops your friend from knocking it to the ground, which is too far away). With your helper in the driver’s seat, go to the prescribed caliper/wheel cylinder (usually the furthest from the master cylinder to start with, but check your Haynes Guide to be sure) and open the bleeder nipple.

Shout “DOWN” and have your helper repeat when they have stepped on the pedal. Now close the nipple, yell “UP” and ask him to do the same when he does not step on the pedal. If they come off the pedal while the bleeder is open, it’s just going to suck air into the system, which is exactly what we’re trying to avoid. 

With each pump down, you will see fluid leak out of the nipple. You’ll also see air bubbles, and that’s good, because we have to get rid of them. Continue to do this until new fluid comes out, with no bubbles. 

5) Filling with brake fluid Keep

on the fluid level in the master cylinder, since you will have to fill it up as you go. Don’t pump the pedal if it goes too low because, again, you’ll just be sucking in air. 

6) Stand Up and Repeat

Do the same on each wheel caliper/cylinder, working closer to the master cylinder as you go. At this stage you should have a decent pedal ‘feel’ which means the pedal should feel firm. This is good, because it means that the brake system is filled with good quality fluid. All you need to do now is check the level in the master cylinder one last time, top it off if necessary, then close it, put the wheels back on and be on your way.

Dispose of used brake fluid carefully. Do not pour down a drain or dump in a vacant lot. Take it to the recycling center of your town hall; you will be directed to the correct place when you enter.


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 refers to fluid in an unsealed container, while wet boiling 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 options and should be changed periodically every 1-2 years depending on your driving habits.

Both of the aforementioned brake fluids attract moisture, which lowers the boiling point.

The brake fluid you choose doesn’t 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 feasible since both are glycol based. Just make sure you don’t mix it with DOT 5 brake fluid.

If you find this post on brake fluid dot 3 helpful to you and you want to know more about car fluid knowledge, please check more on our website Auto Oil And Fluid. Thank you for your interest!

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