Brake fluid dot 4: DOT 3, DOT 4, DOT 5.1 AND DOT 5 BRAKE FLUID A

Brake fluid dot 4: The vehicle’s braking system is a critical system for adjusting the speed of movement of the vehicle, or slowing it down, or stopping it. A brake fluid is a special fluid that ensures effective braking power by transferring the force from the brake pedal to the brake pads. It also has lubricating and anti-corrosion properties that improve the functionality and performance of the brake system and extend its useful life. All of these features make brake fluid selection an especially important task in vehicle maintenance.

Different types of brake fluids are tailored to the requirements of specific brake systems. The most common ones used today are DOT 3, DOT 4, DOT 5 and DOT 5.1 brake fluids.

Are all brake fluids the same? If not, what is the difference between these special fluids?

Here’s what you need to know.


The braking system is a mechanism with extreme operating temperatures. That is why the brake fluid needs to possess certain characteristics in order to meet those conditions. In addition, its main features include:

For effective force transfer

Low viscosity – For compatibility with ABS

Lubricity – To lubricate seals

corrosion – To protect brake system components

High boiling point – To resist high temperatures and not vaporize, which that affect the transfer of braking force.


DOT is an abbreviation for the United States Department of Transportation. Additionally, it indicates the grades of brake fluid specified by the Federal Motor Vehicle Safety Standards (FMVSS), or DOT 3, DOT 4, DOT 5, and DOT 5.1.

Other classification names for the same grades of brake fluid are the J1703, J1704, and J1705 standards published by the Society of Automotive Engineers (SAE) and classes 3, 4, 5, and 5.1 of the International Organization for Automotive Engineering standard ISO 4925. standards.


Brake fluid consists of solvent (60-90%), lubricating agents (5-30%) and additives (2-5%). In addition, brake fluid additives include corrosion inhibitors, which prevent corrosion of metal parts in the brake system, and antioxidants, which prevent solvent breakdown and deposit formation.


DOT 3 uses glycol ether as a solvent and a conventional brake fluid used in vehicles produced up to the 1990s. It is suitable for all brake systems and all driving conditions.

DOT 4 brake fluid uses glycol ether and borate ester compounds that improve its performance compared to DOT 3. Cars built after 2006 use it as standard. Like its predecessor, it is suitable for all braking systems and driving conditions.

DOT 5.1 brake fluid uses glycol ether and borate ester compounds, but has improved performance over silicone-based DOT 5. We can say that it is a silicone-free version of DOT 5 but compatible with types of DOT 3 and DOT 4 braking systems and fluids.

All glycol-based brake fluids must be handled with care as they can damage the paint on the vehicle or motorcycle.


DOT 5 is a silicone-based fluid which makes it incompatible with anti-lock braking systems. Also, DOT 5 cannot be mixed with other types of brake fluids and, unlike other fluids, it does not harm paint.

This fluid is designed for demanding applications such as military vehicles and extremely cold climates.


Yes, but with certain exceptions.

Brake fluids can be mixed only if the fluid you add to the system is of a higher grade.

For example, a braking system that has a DOT 3 brake fluid specification may use a DOT 4 or DOT 5.1 fluid. Also, a system with a DOT 4 can be recharged with a DOT 5.1 without purging the system.

However, never fill a DOT 4 system with DOT 3 brake fluid. DOT 3 will not be able to meet the temperature requirements due to its lower dry and wet boiling points.

Again, do not mix DOT 5 with any other type of brake fluid due to its silicone-based composition.

For optimal brake system and brake fluid performance, it is best to bleed the brake fluid and replace it with fresh.


Dry and wet boiling points are what distinguishes brake fluids. More specifically, they refer to the temperatures at which brake fluid boils under operating conditions.

The dry boiling point refers to the temperature values ​​before the brake fluid becomes contaminated or absorbs moisture. The wet boiling point represents the characteristics of the fluid after a certain time in the system and exposed to atmospheric conditions.

DOT 3, DOT 4 and DOT 5.1 brake fluids. They are hygroscopic, or absorb water, which is why brake fluids have two boiling points. On the other hand, DOT 5 is a highly hydrophobic silicone formulation, that is, it repels water, which makes its boiling points more stable (260 °C dry boiling point and 180 °C boiling point in wet).


Most vehicle manufacturers recommend changing the brake fluid after 2-3 years or 30,000-40,000 km. Therefore, it is important to note that these values ​​differ by manufacturer. That is why it is always a good practice to follow the manufacturer’s recommendations.

What happens if you use the wrong fluids in your car?

It’s not uncommon for people to mix or use the wrong fluids in their cars, and if they do, the results can range from irritating to deadly. Consumer Reports outlines the potential damage you can do to your car—or to yourself—in a story that appears in the November issue.

“Adding antifreeze to the windshield washer reservoir could create a slimy mess,” said David Champion, senior director of the Consumer Reports Auto Test Center in East Haddam, Connecticut. “But a British health study found that filling the tank with just water creates a good breeding ground for the bacteria that cause Legionnaires’ disease. Consumers should check their owner’s manual before filling any fluid under the hood of their car,” Champion said. People should check with a mechanic, or even the folks behind the counter at the local auto parts store, if they have any questions.

The story appears in the November issue of Consumer Reports, which goes on sale Oct. 5. It is also available to subscribers of Updated daily, is the go-to site for the latest car reviews, product news, breaking news blogs and car buying information.

Here’s what could happen if you use the wrong fluids:

Engine oil slippage. The brand of motor oil matters little, but its viscosity grade (10W-30, for example) is important. Use only what is specified in the owner’s manual. Using the wrong oil can result in reduced lubrication and shorter engine life. If the manual says to use synthetic oil, do so. Contrary to what some believe, adding synthetic oil to regular oil will not harm the engine, but there is also no benefit to doing so.

battery fluid. Some car batteries have accessible individual cells that may need to be recharged with a little water to cover the lead plates. Use only distilled water, which does not contain salts or minerals. If tap water is added to a battery’s electrolytic fluid, it can allow minerals in the water to build up on the battery’s internal lead plates, which will reduce battery power and shorten battery life.

Be cool with the water. A car’s cooling system uses a mixture of water and antifreeze; appropriately called coolant, in concentrations (typically 50/50) designed to keep you from freezing on a cold day and boiling over on a hot one. Adding too much water to the mixture can make it more susceptible to freezing and boiling. That can prevent the car from starting when it’s very cold and can cause overheating in warmer weather. Tap water could also cause mineral buildup in the cooling system, reducing its effectiveness.

Adding diesel fuel to the tank of a gasoline car. This will cause the motor to stumble and knock, if it runs at all. Fortunately, diesel pumps have oversized nozzles, so it’s hard to make that mistake. Depending on how much gasoline is added to a diesel vehicle’s tank, it could cause little or no damage to the fuel pump, injectors, and other parts. If the mix-up is caught early enough, a technician can limit the damage by draining the contaminated fuel. Meanwhile, do not run the engine.

Special sauce for your brakes. Brake systems use hydraulic fluid that is specially formulated for that purpose. Replacing transmission or power steering fluid, which are similar to each other, can affect the seals, damage the system, and possibly cause brake failure. Keep in mind that if your brake fluid is low, your vehicle probably needs brake system service anyway. Either the brakes are worn or there is a leak.

Stuck gears. Automatic transmissions should only use fluid specified by the car manufacturer, such as General Motors’ Dexron series or Toyota’s T-type. Using the wrong fluid can cause poor lubrication, overheating, and possibly transmission failure. A mechanic may not be able to reverse the damage, even by flushing the transmission. Adding motor oil or brake fluid by mistake can also destroy your transmission.

Plus no-nos of washer fluid. In addition to creating the perfect environment for deadly bacteria, water doesn’t clean as well as windshield washer fluid and is subject to freezing. The use of household glass cleaners or ammonia can leave foam on the windshield, damage the car’s finish, enter the air intake system and create a potentially harmful cabin environment.

Why does the brake fluid go bad?

Brake fluid is hygroscopic. That means it likes water, like a sponge. In fact, brake fluid is notorious for absorbing water.

Common brake fluid testing devices on the market only test for moisture in the fluid. Here comes the catch: Even if you do everything in your power to prevent moisture absorption, you can still run into problems. Brake fluid can absorb moisture in a number of ways: through the packaging process, while being poured into the reservoir, and even through the brake fluid lines.

How bad is “bad”?

Since brake fluid is hygroscopic, the required tests simulate real world field conditions.

One of the key test parameters is the boiling point of the brake fluid. Wet and dry boiling points are tested to help communicate the quality of a brake fluid. Check out this post for more information, but here’s a quick overview of boiling points:

Dry Boiling Point (ERBP): Measures the boiling point of fresh liquid straight from the bottle before it has been exposed to and contaminated by the humidity.

Wet Boiling Point (WERBP): Measures the boiling point of a fluid after it has had time to absorb moisture from its surroundings, similar to the conditions in which an average vehicle operates (3.7% water contamination, to be exact).

AMSOIL DOT 3 and DOT 4 Synthetic Brake Fluid is specifically designed to perform in wet conditions. With a wet boiling point of 368°F (187°C), it far exceeds the minimum requirements of 284°F (140°C) for DOT 3 fluid and 311°F (155°C) for DOT 4 fluid

How long does brake fluid last?

Under ideal conditions, an unopened bottle of brake fluid will last about two years. It is best to use a new bottle of brake fluid each time you need it because the fluid attracts moisture as soon as it is opened.

Once in your vehicle, the useful life of brake fluid is based on the application in which it is used. Operating conditions, and therefore fluid life, vary depending on the environment, type of equipment, and application.

In high humidity areas, moisture is absorbed through the hoses and seals. Racers change their brake fluid more frequently due to the significantly higher operating temperatures. Up to that point, dry boiling point tends to matter more. That’s why we formulated AMSOIL DOMINATOR® DOT 4 Racing Brake Fluid to deliver an impressive dry boil point of 580°F (304°C).

Can I mix brake fluids?

“DOT 4 and DOT 5.1 are glycol-based brake fluids and are widely used in the automotive and bicycle industries. They are controlled by standards set by the Department of Transportation (DOT), hence the name.

The main difference between these two brake fluids is in their boiling points. Part of the standards that DOT fluid manufacturers must meet are minimum dry and wet boiling points. These are the minimum temperatures your brake fluid must operate at before the brake fluid starts to boil, which can lead to complete brake failure.

Remember, these are only the minimum standards. Brake fluid manufacturers can and often do improve these numbers, and it is possible to find DOT 4 brake fluid with a higher boiling point than some DOT 5.1 fluids on the market.

Since DOT 4 and 5.1 are glycol-based brake fluids, they are compatible with each other, which means they can be easily mixed without damaging the brake system. It is important to never confuse DOT 5.1 (glycol based) with DOT 5 which is silicone based and should never be mixed with any other DOT fluid.

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