Brake fluid DOT 3 vs DOT 4: what’s the distinction and might they be combined?

Brake fluid DOT 3: The brake system of a automotive is a hydraulic system, which implies that it relies on a fluid: brake fluid.

However that isn’t all. The Division of Transportation (DOT) has categorised several types of brake fluid.

This information will deal with the variations between the completely different DOT classifications, emphasizing the distinction between DOT 3 and DOT 4.

The most important distinction is within the dry and moist boiling factors of the brake fluid.

Nonetheless, that comes with different implications that one could not initially consider. Do they have an effect on the efficiency of the brakes? Are you able to combine DOT 3 and DOT 4? Is the upkeep the identical?

We’ll undergo every of those questions and variations, demonstrating how they have an effect on your automotive’s braking system.

Stick with us and skim all about brake fluids beneath!

What’s the DOT classification?

The US Division of Transportation (DOT) has categorised several types of brake fluid to make it simpler and safer for shoppers to make use of the right brake fluid for his or her automobiles.

There are a number of DOT classifications starting from DOT 1 to DOT 5.1. DOT 1 and DOT 2 usually are not made for contemporary automobiles.

There are different classifications for brake fluid as nicely, such because the Worldwide Group for Standardization (ISO) and Society of Automotive Engineers (SAE) classification.

The distinction between DOT 3 and DOT 4

All DOT classes differ in boiling level. There are two kinds of boiling factors, specifically dry boiling level and moist boiling level.

DOT 3 has a dry boiling level of 401°F (205°C) and a moist boiling level of 284°F (140°C).

DOT 4 has a dry boiling level of 446°F (230°C) and a moist boiling level of 311°F (155°C).

What do the completely different boiling factors imply?

When the brakes are in use, they get sizzling, identical to brake fluid.

In case your brake fluid begins to boil, your brake efficiency will probably be compromised, and within the worst case, you gained’t have the ability to cease your automotive.

As you might have seen, there are two kinds of boiling factors, a dry boiling level and a moist boiling level.

The dry boiling level is equal to the boiling level of brake fluid straight from an open container, identical to while you purchased it.

When the brake fluid is within the system and operating, it should develop into contaminated with water by way of micropores within the system.

The moist boiling level is outlined because the boiling level when 3.7% of the water has been absorbed and is now a part of your complete system.

Since water has a decrease boiling level than brake fluid initially does, the extra water combined with the brake fluid, the decrease the boiling level the brake fluid may have.

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

Different Parameters to Think about

The boiling level of brake fluid is because of its composition, which alters not solely the boiling level but in addition different traits.

In any other case, it might make sense to say: “DOT 4 is best than DOT 3”, for the reason that boiling level is increased in DOT 4.

The fact is that there are some benefits of DOT 3. Specifically, because of its composition, DOT 3 is much less hygroscopic, which suggests it should entice much less moisture and water from the surroundings.

Which means that though DOT 3 has a decrease boiling level, it should keep that vary longer than DOT 4, assuming regular driving situations.

Ought to I select DOT 3 or DOT 4?

After realizing all about DOT 3 and DOT 4, it’s possible you’ll be questioning which one you need to use in your automotive.

On the whole, it might be finest to observe the automotive producer’s suggestions. Nonetheless, there are some instances the place one is best than the opposite.

As defined above, DOT 3 brake fluid lasts longer because it doesn’t entice as a lot water as DOT 4.

However, the extra you utilize the brakes and the tougher you brake, the larger the quantity of warmth that’s produced. Since DOT 4 has a better boiling level, it might be the higher selection.

If you happen to drive in a hilly space the place you do a number of braking going downhill or take your automotive to the monitor often, DOT 4 is your best option, however make sure you change your fluid often.

Each DOT 3 and DOT 4 brake fluid needs to be modified often and extra steadily if the brakes have been used greater than regular.

Are you able to combine DOT 3 and DOT 4?

As DOT 3 and DOT 4 brake fluids are glycol based mostly, they are often combined with out drawback, however remember that boiling factors will change.

Make sure to not combine DOT 3 and/or DOT 4 with DOT 5 brake fluid, because the latter is silicone based mostly.

How a lot does it price to vary brake fluid?

Brake and clutch fluid is round £10-15 per litre, and you need to want between 1-2 liters in your automotive relying on the mannequin.

A brake fluid change is an easy job and may take lower than an hour. Nonetheless, if you happen to get the work completed at a foremost vendor you may find yourself paying round £150 for labour, so it makes good sense to do that work your self, with the assistance of a Haynes Handbook.

How Brake Fluid Works

Except you drive a really, very previous automobile, your automotive may have hydraulic brakes. And that’s a superb factor, as a result of hydraulic brake programs are frequent and in addition the best. Nonetheless, they’re solely good if they’re correctly maintained.

Sure, there’s the difficulty of discs and pads, however don’t neglect in regards to the fluid. With out this, your brakes gained’t do something. While you press the pedal, you act on the fluid, which in flip acts on the brakes.

The issue with brake fluid is that it’s hygroscopic. Which means it likes to soak up moisture. The extra moisture that turns into a part of the brake fluid, the decrease the boiling level of the fluid will probably be. So logic would dictate that the longer you permit it on, the much less efficient it turns into.

Can brake fluid go unhealthy in another means?

If there’s a failure in your brake system at any level and the fluid will get too sizzling, you’ll be able to boil it. The decrease the boiling level of the liquid, the simpler it should boil. If the liquid boils, many air bubbles type, permitting the liquid to simply compress. If the fluid might be compressed, it could possibly now not act on the brakes when the pedal is depressed. The pedal turns into ‘tender’ and the brakes now not work.

This may occur after an extended interval of braking, corresponding to happening an extended, steep hill. In truth, this will additionally occur when the automotive is being pushed with gusto (on a race monitor, for instance), the place the brake fluid can overheat and trigger one thing known as brake fade, the place the hydraulic motion of the fluid is much less efficient. .

Take the brake grasp cylinder cap and scent the fluid, if it smells burnt or is an unusually darkish colour, it’s time to vary it.

Your Haynes handbook will information you thru the method of adjusting your brakes and also will cowl what brake fluid your automotive wants. Whereas some fluids are technically interchangeable, it’s at all times finest to be sure you’re filling your brake system with the right materials.

How typically ought to I modify my brake fluid?

Most automobile producers suggest altering brake fluid each 2 years, no matter mileage. The fluid could also be good for longer, however why take the possibility?

Even when the fluid appears positive, you continue to want to vary it. You gained’t see moisture within the fluid, it occurs on a microscopic stage. The very best and most definitive option to know is to seek the advice of your trusty Haynes Handbook.

One factor to contemplate about brake fluid is that even if you happen to’re not altering it, you need to nonetheless verify it often. On the very least, you need to do a cursory inspection each few weeks.

The way to Change Brake Fluid

Each automotive will probably be barely completely different or topic to its personal intricacies. Nonetheless, the overall process stays the identical, which we’ll now see. Earlier than you do, there’s a caveat: discover a second particular person that can assist you. There are one-man bleeder kits on the market, however they’re hardly ever as efficient as having a good friend push the pedal for you. 

1) Drain Outdated Brake Fluid

Take away the cap from the brake grasp cylinder and utilizing a syringe or one thing related, take away as a lot of the previous brake fluid as doable. Put it in an appropriate container. When you’ve eliminated as a lot as you’ll be able to, it’s time to hit the brakes. One of the simplest ways to method that is with the automotive on axle stands, with all 4 wheels eliminated. 

2) Add new brake fluid

. Now that you must get a lint-free material and wipe away any particles that could be within the tank. As soon as that is completed, that you must refill the grasp cylinder with recent brake fluid, as much as the MAX line. Use solely recent fluid from a sealed container. If the container is open, the liquid will soak up moisture from the air. What brake fluid do you want in your automotive? Verify your Haynes handbook or automotive handbook to be sure you are utilizing the right specification fluid. 

3) Rinse the calipers/wheel cylinders

Every brake caliper/wheel cylinder has a bleeder nipple on the rear. You’ll have to loosen this as much as let the previous fluid out, however earlier than you open it up together with your keys, spray it with a little bit of brake cleaner after which penetrating oil to free it up. It will likely be filled with dust from the street. Connect a small piece of rubber hose to the nipple, then submerge the top of the hose into the container that you simply put the previous brake fluid from the grasp cylinder into. 

4) Get Assist

Stick a small block beneath the brake pedal (this stops your good friend from knocking it to the bottom, which is just too far-off). Along with your helper within the driver’s seat, go to the prescribed caliper/wheel cylinder (often the furthest from the grasp cylinder to begin with, however verify your Haynes Information to make certain) and open the bleeder nipple.

Shout “DOWN” and have your helper repeat once they have stepped on the pedal. Now shut the nipple, yell “UP” and ask him to do the identical when he doesn’t step on the pedal. If they arrive off the pedal whereas the bleeder is open, it’s simply going to suck air into the system, which is precisely what we’re attempting to keep away from. 

With every pump down, you will notice fluid leak out of the nipple. You’ll additionally see air bubbles, and that’s good, as a result of we’ve got to do away with them. Proceed to do that till new fluid comes out, with no bubbles. 

5) Filling with brake fluid Maintain

on the fluid stage within the grasp cylinder, since you’ll have to fill it up as you go. Don’t pump the pedal if it goes too low as a result of, once more, you’ll simply be sucking in air. 

6) Stand Up and Repeat

Do the identical on every wheel caliper/cylinder, working nearer to the grasp cylinder as you go. At this stage you need to have a good pedal ‘really feel’ which suggests the pedal ought to really feel agency. That is good, as a result of it implies that the brake system is crammed with good high quality fluid. All that you must do now’s verify the extent within the grasp cylinder one final time, prime it off if crucial, then shut it, put the wheels again on and be in your means.

Get rid of used brake fluid rigorously. Don’t pour down a drain or dump in a vacant lot. Take it to the recycling middle of your city corridor; you can be directed to the right place while you enter.


The distinction between DOT 3 and DOT 4 brake fluid lies within the boiling level of the respective brake fluid.

There are two kinds of boiling factors, moist and dry boiling factors. Dry boiling refers to fluid in an unsealed container, whereas moist boiling refers to when 3.7% of the water has been absorbed.

The boiling level of brake fluid is necessary as it could possibly get highly regarded throughout braking. If it boils, the brakes will develop into ineffective.

Each DOT 3 and DOT 4 brake fluids are viable choices and needs to be modified periodically each 1-2 years relying in your driving habits.

Each of the aforementioned brake fluids entice moisture, which lowers the boiling level.

The brake fluid you select doesn’t actually matter until your automotive producer has specified in any other case. However there are some particular circumstances the place one is best than the opposite.

Both means, mixing DOT 3 and DOT 4 brake fluid is completely possible since each are glycol based mostly. Simply be sure you don’t combine it with DOT 5 brake fluid.

If you happen to discover this submit on brake fluid dot 3 useful to you and also you wish to know extra about automotive fluid information, please verify extra on our web site Auto Oil And Fluid. Thanks in your curiosity!