What is Brake Fluid Made of? Discover the Components Here

You sure have heard of brake fluid. However, do you know what brake fluid is made of? Brake fluid is typically composed of a blend of different chemical compounds. The most common type of brake fluid used in modern vehicles is known as glycol-based brake fluid. This type is further categorized into two main variants: DOT 3 and DOT 4.

Brake fluid is vital to your vehicle’s braking system, ensuring optimal performance and safety. Have you ever wondered what brake fluid is made of? In this article, we’ll dive into brake fluid composition, uncovering its key components and shedding light on its crucial role in maintaining reliable braking power. So, let’s explore the fascinating world of brake fluid and discover what makes it tick.

What is Brake Fluid Made of?

brake fluids

Brake fluid ingredients

Brake Fluid TypeMain ComponentsKey Characteristics
DOT-3Glycol ether– Vapor lock phenomenon with prolonged use
– Decreased wet boiling point due to moisture absorption
– Insufficient metal corrosion resistance
DOT-4Glycol ether– Higher wet boiling point than DOT-3
Boric acid ester– Enhanced metal corrosion resistance
DOT-5.1Glycol ether– Similar characteristics to DOT-4 with improved properties
Boric acid ester
– Suitable for high-performance and heavy-duty applications

Brake fluid is primarily made of various chemicals, with the exact composition depending on the type of brake fluid. The two most common types of brake fluid are glycol-based (DOT 3, DOT 4, DOT 5.1) and silicone-based (DOT 5).

  1. Glycol-based Brake Fluid (DOT 3, DOT 4, DOT 5.1):
    • Glycol Ether: The main ingredient in glycol-based brake fluids is glycol ether, such as ethylene glycol or diethylene glycol. It is the primary hydraulic fluid, transmitting the pressure from the brake pedal to the brake components.
    • Borate Ester: Glycol-based brake fluids also contain a borate ester compound, which acts as an additive to improve the fluid’s properties, such as its boiling point and corrosion resistance. Depending on the specific brake fluid grade, different types of borate esters may be used.
    • Corrosion Inhibitors: To protect the metal brake components from corrosion, glycol-based brake fluids incorporate corrosion inhibitors. These inhibitors help prevent the formation of rust and corrosion inside the brake system.
    • Anti-foaming Agents: Brake fluids may include anti-foaming agents to prevent the formation of air bubbles or foam, which could negatively impact brake performance.
  2. Silicone-based Brake Fluid (DOT 5):
    • Silicone Polymer: Silicone-based brake fluid, also known as DOT 5, is composed mainly of silicone polymer. This brake fluid type differs from glycol-based fluids as it does not absorb moisture from the air.
    • Additives: Silicone-based brake fluid may contain various additives to improve its performance characteristics, such as viscosity and resistance to high temperatures.

It’s worth noting that DOT 5 silicone-based brake fluid is not compatible with glycol-based brake fluids and should not be mixed. It is essential to use the brake fluid specified by the vehicle manufacturer and follow their recommendations for the appropriate type and grade of brake fluid to ensure proper brake system operation and safety.

The Invention Of Brake Fluid

The brake fluid composition consists of a base fluid, a metal corrosion inhibitor, and an antioxidant. The base fluid comprises 70 to 80 wt. % of glycol ether, 18 to 28 wt. % of a boric acid ester, and 0.8 to 1.2 wt. % of a silane-type stabilizer. This composition aims to prevent sediment formation of boric acid from the boric acid ester, resulting in increased metal corrosion inhibition and extended usability.

To achieve the desired properties, specific ratios of glycol ether and boric acid ester are used in the base fluid. Examples of glycol ether include triethylene glycol monomethyl ether, polyalkylene glycol, polyethylene propylene glycol monomethyl ether, and polyethylene glycol monobutyl ether.

Triethylene glycol monomethyl ether is used in the range of 20 to 30 wt. %, while polyethylene glycol is used in the range of 5 to 10 wt—%, with an average molecular weight ranging from 700 to 4,000. Polyethylene glycol monobutyl ether is preferred in the range of 20 to 30 wt. %, and polyethylene propylene glycol monomethyl ether is used in the range of 15 to 30 wt. %.

The boric acid ester content is particularly important, ranging from 18 to 28 wt. %. Below 18 wt. %, the physical properties of the brake fluid become poor while exceeding 28 wt. % can lead to metal corrosion due to sediment formation and increased manufacturing costs.

A metal corrosion inhibitor is included to prevent corrosion and weight change of metal parts. The inhibitor typically consists of a mixture of phosphate, triazole, and amine. The content of the metal corrosion inhibitor ranges from 1.0 to 2.1 wt. %, with phosphate comprising 0.5 to 1.0 wt. % of the total weight. Triazole and amine are used in the range of 0.2 to 0.5 wt. % and 0.3 to 0.6 wt. %, respectively.

An antioxidant is added in the range of 0.3 to 0.6 wt to prevent oxidation of the base fluid’s alkyl groups. %. Dibutylhydroxy toluene is a preferred antioxidant choice. The brake fluid composition also includes a silane-type stabilizer, which prevents metal corrosion caused by boric acids resulting from the hydrolysis of boric acid esters. The content of the stabilizer ranges from 0.8 to 1.2 wt. %.

When the content is below 0.8 wt, %, boric acid formation occurs, and exceeding 1.2 wt. % is not recommended due to the potential corrosion of metal parts. A preferred stabilizer is 3-diethanol aminopropyl silane.

In conclusion, the brake fluid composition described in the present invention, which incorporates glycol ether, boric acid ester, a metal corrosion inhibitor, an antioxidant, and a silane-type stabilizer, offers benefits such as cost-effectiveness, high wet boiling point, excellent corrosion inhibition, and extended usability.

Types of Brake Fluid And Its Characteristics

Brake FluidDry Boiling PointWet Boiling PointViscosity at -40 °CViscosity at 100 °CPrimary Constituent
DOT 2190 °C (374 °F)140 °C (284 °F)Castor oil/alcohol
DOT 3205 °C (401 °F)140 °C (284 °F)≤ 1500 mm²/s≥ 1.5 mm²/sGlycol ether
DOT 4230 °C (446 °F)155 °C (311 °F)≤ 1800 mm²/s≥ 1.5 mm²/sGlycol ether/borate ester
DOT 4+230 °C (446 °F)155 °C (311 °F)≤ 750 mm²/s≥ 1.5 mm²/sGlycol ether/borate ester
LHM+249 °C (480 °F)249 °C (480 °F)≤ 1200 mm²/s≥ 6.5 mm²/sMineral oil
DOT 5260 °C (500 °F)180 °C (356 °F)≤ 900 mm²/s≥ 1.5 mm²/sSilicone
DOT 5.1260 °C (500 °F)180 °C (356 °F)≤ 900 mm²/s≥ 1.5 mm²/sGlycol ether/borate ester
DOT 5.1 ESP260 °C (500 °F)180 °C (356 °F)≤ 750 mm²/s≥ 1.5 mm²/sGlycol ether/borate ester
ISO 4925 Class 3205 °C (401 °F)140 °C (284 °F)≤ 1500 mm²/s≥ 1.5 mm²/s
ISO 4925 Class 4230 °C (446 °F)155 °C (311 °F)≤ 1500 mm²/s≥ 1.5 mm²/s
ISO 4925 Class 5-1260 °C (500 °F)180 °C (356 °F)≤ 900 mm²/s≥ 1.5 mm²/s
ISO 4925 Class 6250 °C (482 °F)165 °C (329 °F)≤ 750 mm²/s≥ 1.5 mm²/s
ISO 4925 Class 7260 °C (500 °F)180 °C (356 °F)≤ 750 mm²/s≥ 1.5 mm²/s

What is the importance of brake fluid?

Brakes are one of the most important safety features of your vehicle. They help you stop safely when needed. You could have serious injuries or even death if they do not function properly.

Brake fluid plays a vital role in ensuring that your brakes work properly. Without it, your brakes would not be able to generate enough pressure to slow down your car or truck. This means you would have no way to stop without brake fluid in your vehicle’s master cylinder.

When you apply the brakes, the brake pads press against the rotors of your wheels. As these pads move across the surface of the rotor, the friction causes heat to build up between them. When the temperature gets too high, the brake fluid inside the master cylinder begins to boil.

How often should I change my brake fluid?

It’s best to change your brake fluid every 12 months, but if you drive your car hard at the track, you should replace it more often. Brake fade, and corrosion can occur with water-contaminated brake fluid, so it’s a good idea to flush and bleed your system every time you take your car to the track, even if you don’t drive it hard there.

Bleeding the brakes entails circulating fluid to remove air from the brake lines, while flushing is intended to remove contaminants and/or old fluid from the lines. If you have doubts about your ability to do it yourself, seek professional assistance.

symptoms of bad brake fluid

When brake fluid deteriorates or becomes contaminated, it can exhibit several symptoms indicating that it needs to be replaced. Here are some common symptoms of bad brake fluid:

Soft or spongy brake pedal: If the brake fluid becomes contaminated with air bubbles or moisture, it can cause a soft or spongy feeling in the brake pedal. This can lead to a decrease in braking performance and longer stopping distances.

Increased brake pedal travel: If the brake fluid is old or contaminated, it may cause the brake pedal to have increased travel or require more force to engage the brakes. This can result in a delayed response when applying the brakes.

Reduced braking efficiency: Bad brake fluid can lead to reduced braking efficiency, causing longer stopping distances. This can be especially noticeable during sudden or emergency braking situations.

Brake fade or overheating: Contaminated brake fluid can lower the fluid’s boiling point, leading to brake fade or overheating during prolonged or aggressive braking. This can result in a loss of braking power and an increased risk of brake failure.

Strange noises or vibrations: In some cases, deteriorated brake fluid can cause the brakes to make unusual noises, such as squealing, grinding, or vibrating. These noises may indicate that the brake components malfunction due to contaminated brake fluid.

Corrosion and brake component damage: If the brake fluid contains moisture or other contaminants, it can cause corrosion and damage to brake system components over time. This can lead to leaks, brake line failures, or even brake system malfunctions.

If you experience any of these symptoms, it is essential to have your brake fluid inspected and, if necessary, replaced by a qualified mechanic or technician. Regular maintenance and periodic brake fluid flushes can help ensure optimal brake performance and safety.

What are the consequences of driving with low brake fluid?

Regarding brake fluid, most drivers know just enough to be dangerous. They know that it’s important, and they know that it needs to be checked regularly. But many drivers don’t realize that brake fluid plays a vital role in the braking system. When brake fluid levels are low, it can cause a number of problems.

For one thing, it can cause the brakes to feel “spongy,” making it harder to stop the car. Low brake fluid can also lead to leaks in the braking system, eventually resulting in complete failure.

In short, driving with low brake fluid is like playing Russian Roulette with your life – sooner or later, you’ll end up in a situation where you wish you had brakes. So if you want to stay safe on the road, keep an eye on your brake fluid levels.

What are some tips for changing brake fluid?

There are a few things to keep in mind when changing your brake fluid:

Brake fluid needs to be changed at designated intervals. If you don’t change it at the correct intervals, it can clog systems and cause damage.

Always handle brake fluid with care. It is highly corrosive and damaging to paint and other coatings if spilled.

When choosing a brake fluid, always defer to factory specifications. There are many brake fluid types, so ensure you get the right one for your car.

The average car owner will need DOT 3, DOT 4, or DOT 5.

To replace the brake fluid, you will need to remove the wheel and axle assembly. Make sure you have all of the necessary tools before getting started.

Click here to know how often should you change brake fluids.


What does brake fluid do in a car?

Brake fluid is an important part of the braking system in a car. It performs two main functions: Applying the brake pressure to the brake pads, which causes them to squeeze and slow down the rotation of the wheels—acting as a medium for transferring this applied pressure from the master cylinder to each wheel’s hydraulic system.

What are the symptoms of bad or old brake fluid?

Brake fluid is a critical component of your vehicle’s braking system. It is responsible for transferring the force from the brake pedal to the brake pads and eventually to the rotors. However, if there are Burning smell when braking, Reduced stopping power, or a Slippery pedal, then it’s time to change the brake fluid.

How often should brake fluid be changed?

Brake fluid is a hydraulic fluid that is used to provide the necessary pressure for the brake system. It can be found in both disc and drum brakes. Brake fluid should be changed every year or every 20,000 miles of use, whichever comes first.

What are the leading types of brake fluid in the market?

Brake fluid is a type of hydraulic fluid used in a vehicle’s braking system. It is mainly composed of organic compounds, such as esters. The two most common types of brake fluid are petroleum-based and silicone-based, but there are also other types that are based on synthetic fluids, glycols, or water.

How should brake fluid be disposed of for safety and environmental reasons?

Brake fluid contains a toxic substance that can damage the environment. It is also dangerous to humans. The brake fluid should be disposed of to prevent it from harming the environment and people. The brake fluid should be mixed with kerosene before disposal. This will prevent it from evaporating into the air and harming the environment. Pouring the brake fluid down a drain or on the ground is not recommended because it can contaminate groundwater and harm animals in these areas.


brake fluid is a crucial component of a vehicle’s braking system, responsible for transmitting the force applied to the brake pedal to the brake calipers or wheel cylinders, enabling effective stopping power. Brake fluid is a specialized hydraulic fluid formulated to meet the demanding requirements of the braking system. Now you have a clear concept of brake fluid, so now you can take necessary steps whenever you need.

Rahat Hossain