DOT 3 vs DOT 4 vs DOT 5: What are The Differences?

Brake fluid transmits pressure from the brake lever to the brake pads. These fluids need to be non-compressible for them to transfer the force effectively. The best brake fluid should lubricate the calipers, resist corrosion, and have a high boiling point. Brake fluid comes in four types. There are DOT 3, DOT 4, DOT 5.1, and DOT 5 brake fluids. Brake fluids are exposed to an average amount of heat because brakes convert kinetic energy into heat.

However, it can only withstand a certain amount of heat before it starts to boil. When it boils, it forms air bubbles, making it compressible and affecting how the braking system works. DOT 3 and DOT 4 are the most common brake fluids used in automobiles.

DOT5 fluid is more heat-resistant than its DOT3 and DOT4 counterparts. So, it has a higher compressibility than glycol. Furthermore, DOT3 and DOT4 can be mixed, but DOT5 is not compatible with either.

This discussion comparing DOT 3, DOT 4, and DOT 5 brake fluids will help you make the right decision about which to use on your vehicle.

DOT Brake Fluid Explained

The most commonly used brake fluid today is DOT fluid. All the DOT fluids apart from the DOT 5 are made of the poly-glycol base. Glycol-based brake fluids are mixtures of up to 10 ingredients, resulting in the final products. The substances can be broken into four major components, which include:

DOT brake fluid meets stringent specifications and standards set by the Department of Transportation and the Society of Automotive Engineers. DOT stands for the Department of Transportation. The standards focus on maintaining the performance of brake fluids in both low and high temperatures. They also specify the minimum boiling temperatures to which the manufacturers of the fluid must adhere.

There are different types of DOT brake fluids. The DOT brake fluid types include DOT 5.1, DOT 5, DOT 4, DOT 3, and DOT 2. The main thing that separates the various classes of DOT brake fluids is their boiling points. Boiling points refer to where the brake fluid will begin to boil or vaporize in the brake system. It is mainly caused by heavy, prolonged braking, which adversely affects the performance of the entire brake system.

What is DOT 3 Fluid?

DOT 3 brake fluids are petroleum/non-mineral-based, non-silicone, and fully synthetic brake fluids designed for use in a wide range of clutch and brake applications. It is made by mixing alcohol and glycerin. The brake fluid uses polyethylene glycol ether technology for high performance.

The main advantage of DOT 3 brake fluid is its high boiling point, which provides safe, consistent performance at high braking pressure. This is designed to withstand high temperatures without boiling and cold temperatures without thickening. And it can withstand high temperatures of up to 250° C.

The rubber component swells in DOT 3 brake fluid, which is minimal, thus reducing fluid loss and leakage. It also provides excellent corrosion protection, improving the reliability and lifespan of components in the brake system.

This brake fluid is suitable for top-ups or refills of clutch and brake systems in heavy-duty commercial vehicles, motorcycles, 4WDs, and passenger cars. You can also use it on agricultural, construction, and mining equipment.

Use this brake fluid when the vehicle manufacturer specifies DOT 3. The fluid is naturally hygroscopic and absorbs water from the air, reducing the product’s effectiveness. Once you open the brake fluid bottle, make sure you reseal the cap tightly to prevent water contamination.

You should change the brake fluid according to the manufacturer’s service specifications. You are advised not to mix the DOT 3 brake fluid with Mineral or Silicone DOT 5. Always wear protective gear when using the brake fluid to prevent contact with paint, varnish, or skin.  If your skin comes into contact with brake fluid, wash it immediately with water.

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What is DOT 4 Brake Fluid?

DOT 4 brake fluids are glycol ether-based and have an addition of borate esters, which improves their performance. Some of the properties that borate esters improve include wet- and dry-boiling points. DOT 4 brake fluid has a higher, more stable boiling point than DOT 3. However, when it begins to absorb water, the boiling point falls more rapidly than that of the DOT 3 brake fluid.

DOT 4 should have a minimum wet boiling point of 155°C and a minimum dry boiling point of 230 °C, according to the set standards. Use the DOT 4 brake system only if it is your preferred brake fluid or if the car manufacturer has specified it. You are advised not to use this brake fluid for lower or upper-grade vehicles.

It is recommended to replace the DOT 4 brake system every 2 years to maintain maximum safety and performance. The lifetime of the brake system is reduced when water contaminates it. DOT 4 brake fluid absorbs moisture through the brake hoses. The brake fluid heats up as you continue to use the brake system.

The boiling point of the brake fluids will decrease as they continue to absorb water from the air. If you fail to change brake fluid regularly, the wheel cylinders and master cylinder rubber parts will start to corrode. You can mix DOT 4, DOT 3, and DOT 5.1 without reducing performance or damaging the braking system, since they are glycol-based.

However, even if it is safe to mix them professionally, I would not advise you to combine them. DOT 4 is an alternative to DOT 3, but not the other way round. Make sure you have flushed all the brake fluid from the system before you fill the reservoir with another type of brake fluid.  This brake fluid can withstand temperatures up to 311 degrees and has a viscosity of 1800 at 40 F.

What is DOT 5 Brake Fluid?

DOT 5 brake fluids are silicone-based and are used in most modern cars. It is more expensive than others because it has a high byline point. It has a dry boiling temperature of 356 degrees. Most new brake rotors are thin and small, so they dissipate a lot of heat. This silicon-based brake fluid is ideal for this type of car since it can withstand high temperatures.

A DOT 5 brake fluid does not absorb moisture, and it does not cause any harm to painted surfaces. This brake fluid acts as a weather barrier to the brake system. DOT 5 can replace DOT 3 and DOT 4, but you should not mix it with any other brake fluid. Silicon-based brake systems are more difficult to bleed.

However, once they bleed, a brake system filled with DOT 5 brake fluid will resist rust better and remain effective longer than other formulations. This brake fluid is also lighter in cold temperatures. It weighs around 900 at 40 F. The viscosity of DOT 5 is much more stable in extreme temperatures. It is ideal, can be used with most external boots, and is compatible with all components of standard brakes.

DOT 5 brake fluids also have some drawbacks. It expands significantly when hot, and the additives can vaporize at moderate temperatures, thus increasing the spongy feel. This fluid is incompatible with systems that have used glycol-based fluids before, unless you flash it and have the seal replaced. It is also not compatible with anti-lock brakes because the silicone fluids are more viscous. Unlike glycol fluids, which start compressing as they near their boiling points, silicone brake fluids begin to compress at 300-350 degrees Fahrenheit.

What is DOT 5.1 Brake Fluid?

Most people think that DOT 5.1 is a modification of DOT 5, but that is not the case. DOT 5.1 is silicone-based and is a combination of a borate ester and a polyalkylene glycol ether. DOT 5.1 brake fluid sustains more heat than other DOT brake fluids without any mushy feelings. It can withstand higher dry and wet boiling temperatures and provides superior performance compared to all other DOT brake fluids.  You can mix it with all the glycol-based brake fluids, but it is not compatible with DOT 5.

This fluid has a dry boiling temperature of around 275 Degrees Celsius, which is higher than that of a racing fluid. Another advantage of DOT 5.1 is that it is convertible with rubber formulations. It is also compatible with DOT 3 and DOT 4. It is therefore easy to upgrade from DOT 3 and DOT 4.

On the downside, DOT 5.1 will absorb water, it eats paints, and it is also more expensive than DOT 3 and DOT 4. This brake fluid is also challenging to find in some auto parts stores. The brake fluid is not compatible with DOT 5; you should never mix it with DOT 5 for any reason.

DOT 3 vs. DOT 4 Brake Fluid: Which One Is The Better Brake Fluid?

DOT 3 brake fluid is a more popular choice for cars and trucks. It is low-cost and a standard option for average cars, where the driver is unlikely to engage in vigorous braking. DOT 4 is also gaining popularity due to its compatibility with traction control and anti-lock braking systems. It is ideal for police and racing cars that need vigorous braking and can use it regularly. Below are some other differences between the DOT 3 and DOT 4:

01. Chemical Structure

The chemical components in DOT 3 and DOT 4 differ slightly. DOT 3 is made by blending polyalkylene and ether, while DOT 4 is a mixture of borate and glycol. The glycol ether holds up well in wet, hot conditions, making it a perfect choice for regular vehicles. DOT 4, on the other hand, has a high water tolerance and is stable at high temperatures.

02. Boiling Point

The major difference between DOT 3 and DOT 4 fluids is their boiling points. DOT 3 has a lower boiling point and thus absorbs water more readily. It therefore boils faster under rough, hard braking, which could damage the braking system. DOT 4 has a higher boiling point. It is an alternative to DOT 3 brake fluid, but DOT 3 cannot be used in place of it.

03. Boiling Capacity

DOT 3 brake fluid has a great dry and wet boiling capacity. It means it will still work better when exposed to open air and water than DOT 4. DOT 4 braking fluid, on the other hand, has a better dry boiling point but a poor wet boiling point.

Difference between DOT 3, DOT 4, and DOT 5 Brake Fluids

FAQs

What Happens If You Mix DOT 3 And DOT 4?

Mixing DOT 3 and DOT 4 will give you an unpredictable boiling performance, which should not cause any harm. These brake fluids are compatible with one another, so there is no problem mixing them. The difference between the two is minimal. However, professionally, I would advise you to use one or the other instead of mixing.

When Should I Use DOT 4 Brake Fluid?

You can use DOT 4 brake fluid if you want to upgrade from DOT 3. DOT 4 has a high boiling point, making it ideal for cars that engage in vigorous braking, such as motorcycles, racing cars, and police vehicles. It is also compatible with cars using anti-lock braking systems. It is advisable to use DOT 4 brake fluid if the car manufacturer recommends it.

Are All DOT 4 Brake Fluids The Same?

All DOT 4 fluids are synthetic brake fluids, and their chemical composition is the same. All DOT 4 brake fluids have the same wet and dry boiling points, and thus, you can use any, depending on your preference. They are a blend of borate ester and glycol.

Does DOT 5 Absorb Water?

DOT 5 does not absorb water, which gives it a high boiling point. Since it is silicone-based, it prevents moisture from entering the brake system. If any moisture gets in the brake system, it collects in pockets, which later boil off or freeze. Moisture can lead to failure or damage of the brake system.

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Conclusion

Brake fluid is mandatory for every type of vehicle. It does not matter if you are driving an economy car, truck, or motorcycle. The function of brake fluid is to lubricate the vital components of the braking system. The brake fluid enables the pistons in the braking system to compress the rotors, slowing the car.

There are various brake fluid types, as we saw in the above DOT 3 vs. DOT 4 article. The type of brake fluid you choose will depend on the type of brake system you are using. Most vehicle manufacturers will also recommend the brake fluid you should use.

It is very important to follow the recommendations in your car manufacturer’s manual. You can upgrade from DOT 3 to DOT 4 to DOT 5.1. These brake fluids are glycol-based. DOT 5 brake fluids are silicone-based, and you should not mix them with other fluids. This brake fluid was mainly made for military vehicles. Ensure that you use the right type of brake fluid for your vehicle to maintain safety.