DIY

What is Brake Fluid and What Does it Do?

Reading Time: 4 minutes

Every driver knows what brake fluid is, and you’ve probably topped up once or twice already. But have you ever wondered what it does or how it functions inside the system? What are its components? What are the differences between varying types and composition? Hold your breath as we’re about to dive deep into a car’s braking fluid.

mechanic refilling brake fluid of a car
Brake fluid transfers the energy from pedal depression to the brake calipers.

What Does Brake Fluid Do?

Brake fluids usually contain glycol ether or diethylene glycol and must have a constant viscosity under a wide range of temperatures, from extremely hot to freezing cold. The main purpose of this fluid is to transfer the energy from pedal depression to the brake calipers.

The brake fluid must also have a high boiling point to resist the extreme temperatures created during braking.

Brake fluid acts as a hydraulic agent—more on this below. It is also formulated to protect the metal brake components from corrosion and moisture. This is achieved by adding various additives, although some brake fluids come pre-mixed.

Why Does a Car Need Brake Fluid?

Nearly all cars use pressurized brake fluid to activate the brakes. Because the braking system uses a liquid to create force, it’s known as a hydraulic system. Many modern vehicles have two hydraulic systems: the power steering system and the brake system.

Although the fluids in these systems both work in very similar ways, the two differ in application. The brake fluid needs to have a higher boiling point in order to avoid vaporization and has to be incompressible for the system to properly work. Without the brake fluid, depressing the brake pedal will not activate the brake caliper or wheel cylinder.

What is a Hydraulic Braking System?

In order to understand brake fluid, you need to know how the brake hydraulic system works. When you apply the brake pedal, a device called the master cylinder transfers force from your foot to the brake fluid. From there, the pressurized fluid flows to a brake caliper (or wheel cylinder in a drum brake system) at each wheel.

The fluid forces the caliper to squeeze the brake pads against the rotor, creating the friction needed to bring your vehicle to a stop.

close up shot of a car's brake pedal
The master cylinder transfers force to the brake fluid when you step on the brake pedal.

DOT in Brake Fluid: What Does DOT Mean?

DOT stands for the US Department of Transportation, which issues the standards for all motorized vehicle brake fluids in the country. To classify, brake fluids are rated by the base component they are made of.

There are currently four hydraulic brake fluid types: DOT 3, DOT 4, DOT 5, and DOT 5.1.

DOT 3 Brake Fluid

DOT 3 brake fluid is glycol-based and is the most common type of fluid. This is because it is relatively cheap due to its poor boiling point.

DOT 4 Brake Fluid

Like DOT 3, this type of brake fluid is glycol-based. It is one step above DOT 3 with its higher boiling point and moisture resistance properties.

DOT 5 Brake Fluid

DOT 5 brake fluid, meanwhile, is silicone-based and works well at resisting moisture. DOT 5 also doesn’t harm your car’s paint, but the downside is that it can’t be mixed with the glycol-based DOT 3 and DOT 4.

This fluid is usually tinted violet to distinguish it from DOT 3, DOT 4, and DOT 5.1.

DOT 5.1 Brake Fluid

DOT 5.1 is a non-silicone-based polyglycol fluid. Like DOT 5, it works well at resisting moisture.
It is made with a polyglycol base that gives it a dry boiling point that’s over 500 (sometimes 600) degrees Fahrenheit.

The good thing about this type of hydraulic fluid is that you can mix it with DOT 3 and DOT 4 (when the vehicle manufacturer specifies that doing so is safe).

To learn more about the different boiling points of brake fluid, read on.

Brake Fluid Boiling Point: Dry and Wet

Brake fluids have varying boiling points. The lowest boiling point belongs to DOT 3 while the highest one can be found on DOT 5.1 fluids.

Brake fluid has two boiling point ratings, which are dry and wet. The dry boiling point pertains to the boiling point of fresh brake fluid in an unopened container. The wet boiling point, on the other hand, refers to the fluid’s boiling point after it has accumulated water, which begins at the start of its service life.

Brake Fluid TypeDry Boiling PointWet Boiling Point
DOT 3401 degrees Fahrenheit284 degrees Fahrenheit
DOT 4446 degrees Fahrenheit311 degrees Fahrenheit
DOT 5500 degrees Fahrenheit356 degrees Fahrenheit
DOT 5.1500-600 degrees Fahrenheit356+ degrees Fahrenheit

When to Change Brake Fluid

There are varying opinions on when to change the brake fluid, and there’s no exact schedule for it. The answer may also depend on the model of the car you are driving. However, many automakers do not include a brake fluid service as part of the recommended maintenance schedule.

As such, you may not be able to determine when the fluid should be changed by looking in your owner’s manual.

To help you figure out when you should change your brake fluid, a good rule of thumb would be to regularly check its clarity, consistency, and level. There are also test strips you can purchase to check the condition of the fluid. Simply dunk one of the strips into the brake fluid reservoir and follow the instructions on the bottle to determine the condition of your car’s fluid.

Most experts recommend replacing your brake fluid on a regular basis to avoid moisture buildup. Moisture in the system can reduce the boiling point of the brake fluid, leading to a potentially dangerous decline in braking ability.

topping up a car's brake fluid
Regularly check your brake fluid’s clarity, consistency, and level to know when to change it.

Before you change or top off your brake fluid, you must know which type is already in your car. Consult your owner’s manual to determine the correct type of fluid to use.

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In the Garage with CarParts.com is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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