What Does Brake Fluid Do?

What Does Brake Fluid Do?
Ulianenko Dmitrii/Shutterstock

One of the things we often take for granted is brake fluid. No one sits around the dinner table discussing its benefits or merits, and why should they? But think about it for a moment and you might wonder how it allows even the frailest of drivers to stop a two-ton vehicle with one foot. What does brake fluid do that makes this possible?

What is brake fluid, and what does brake fluid do?

Brake fluid is a hydraulic fluid that moves components within the braking system. When you press down on the brake pedal, the compression builds pressure in the braking system, which forces fluid down to your brakes. That pressure squeezes the brake pads onto the brake rotor to slow you down. The brake fluid must operate perfectly at high pressures and temperatures because of braking’s high-stress, high-friction nature. Because of its harsh working environment and essential use, you should regularly check the brake fluid to try and ensure proper braking performance in every condition.

What are the types of brake fluid?

There are four types of brake fluid: DOT 3, DOT 4, DOT 5 and DOT 5.11. The “DOT” stands for “Department of Transportation,” which sets safety regulations for every transportation type in the United States. The three letters mean that the DOT has set, tested and provided safety regulations for the brake fluid’s performance.

None of the four fluid types are the same. There are differences in composition between what you would use in your perfectly normal SUV and what would be used in a 500 horsepower sports truck. For example, cars with anti-lock braking systems (ABS) use DOT 4 and 5 brake fluids due to extra additives that improve their ability to take the heat. A higher DOT rating doesn’t necessarily mean it’s better for your car. Instead, use the type specified by your owner’s manual.

Let’s do a deeper dive into each type of brake fluid.

DOT 3

DOT 3 is the most popular and has been used for a long time. Fresh DOT 3 brake fluid is glycol-based and has a boiling point of 401 degrees Fahrenheit. Once it has been in your brake system for a few years, the boiling point drops to 284 degrees Fahrenheit. If you drive in hilly terrain, tow, or take part in track days, this will happen sooner than normal. Since glycol-based brake fluids are highly corrosive and can remove paint, you should be very careful while using them. Clean up immediately using soap and water or a simple degreaser.

DOT 4

DOT 4 is beginning to be used more widely by vehicle manufacturers, primarily in Europe. Manufacturers have found that it’s more compatible with cars using anti-lock braking systems. Fresh DOT 4 fluid’s boiling point starts at 446 degrees Fahrenheit. There are additional additives in DOT 4 that help reduce the caustic acids that can form from moisture.

While DOT 3 and 4 are technically capable of being mixed, it’s not recommended because DOT 4 is twice the cost but offers little added benefit. If you need it, there are several different types of DOT 4:

- DOT 4: Used in some Euro and domestic vehicles. DOT 4 absorbs more water from the air over time and needs to be changed more frequently. But it has a higher boiling point, making it safe for higher temperatures.

- DOT 4 Plus: Used in Mercedes and Volvo automobiles. Specifically designed for Mercedes-Benz ABS systems. It has a higher boiling point temperature for performance applications and a shorter change interval of two years.

- DOT 4 Low Viscosity: Used in BMW vehicles. DOT 4 LV has a lower boiling temperature (less performance), but does not attract moisture as much as its competition.

- DOT 4 Racing: For high-temperature applications. DOT 4 Racing has a boiling point of 590 Fahrenheit when new and 518 Fahrenheit when used. It’s suitable for the most demanding motorsports applications.

DOT 5

DOT 5 is a silicone-based brake fluid with a boiling point of 500 degrees Fahrenheit. It is often purple in color in order to differentiate itself from DOT 3 and 4’s amber color and, because it’s silicone-based, it won’t absorb water like the glycol-based brake fluids. Moisture does form bubbles in DOT 5 and they are difficult to remove. Because of that, DOT 5 isn’t recommended for ABS systems. It also can’t be mixed with any other fluid and is four times more expensive than DOT 3.

DOT 5.1

DOT 5.1 is a glycol-based brake fluid with a boiling point similar to DOT 4 racing brake fluids. Usually clear to amber in color, it is technically able to be mixed with DOT 3 or 4, but it is again not recommended due to the costs involved—DOT 5.1 is approximately 14 times more expensive than DOT 3.

How do I find out what kind of brake fluid I have?

If you’re not sure about the type of brake fluid in your car, the DOT specifications are typically written on the brake fluid reservoir cap under the hood and near the driver’s side firewall. You should also be able to locate it in your owner’s or service manual. But let’s assume that you don’t have the required documentation handy, or you suspect it may have DOT 5 in it instead of DOT 4. Can you otherwise confirm?

Yes, you can. Suck out a little bit of brake fluid with a syringe and put it in a glass. Then get a bit of water and pour it into the same glass. Stir them together, and if the water mixes with brake fluid, it’s a glycol-based DOT 3 or 4 fluid. If the liquid has bubbles in it and doesn’t mix, similar to water with olive oil, it’s a silicone-based DOT 5 fluid. DOT 5 fluid will also be purple in color instead of the glycol-based, amber-colored brake fluid.

Do I need to change my brake fluid?

Although there is no set time to change the brake fluid in your vehicle, it benefits from periodic changes. The timing of when to change fluid depends on the car and its driving conditions. A good rule is to check the brake fluid during regular oil changes, follow your manufacturer’s recommendations and expect to change it every four to five years at least.

Signs you need to change your brake fluid

Owner’s manuals and online information from the manufacturer will give you guidance on recommended maintenance. However, you can change brake fluid anytime you choose. If you regularly pull heavy trailers, then you’ll want to change the fluid more frequently. Even driving in stop-and-go commuter traffic can be classified as harder than regular use. If you’re looking for signs, here are a few that will let you know that your brake fluid may need to be changed.

  • You have to press the pedal down farther than usual to get the car to stop. This can also happen if your front brake pads’ friction material has worn down to the point they need replacing. Either way, it’s time to see what’s wrong.
  • You have to press the brake pedal down more than once. The need to pump the brakes lets you know there isn’t enough brake fluid in the system to stop at a safe distance. The need to pump your brake pedal more than once slows down your reaction time and can cause an accident. You should carefully drive to a location that can inspect your braking system.
  • The brake warning light on your dashboard lights up. If this happens, you should carefully drive your car to the closest automotive professional and have it checked out as soon as possible.

What if you don’t ever change your brake fluid?

It’s not unusual to go years without changing brake fluid. You may think that everything is OK because you don’t feel like there’s a problem, but that doesn’t mean there aren’t issues boiling, sometimes literally, that could cause an accident. Here are some things that happen when a brake fluid change is put off.

Moisture contamination

Generally, you should never open your brake fluid reservoir cap unless you’re changing the fluid. Glycol-based brake fluid is hygroscopic, meaning it’ll absorb any moisture that’s present. The brake fluid system’s sealed environment protects it against moisture and air.

But even in this system, age will cause seals to slowly deteriorate, letting air and moisture contamination creep in. Moisture can lower the boiling temperature and turn into a gas which causes vapor pockets to form. This will cause your pedal to feel spongy, and your braking will lose its effectiveness.

Debris buildup

As your brake fluid ages, it will also be contaminated with debris. Seals and hoses naturally corrode in the fluid over time as they age. Particles from metal components can also corrode as brake fluid ages and takes in moisture. Keep the brake fluid reservoir cap closed so that things won’t make their way into the system.

Although you can find the bulk of the debris suspended inside your brake fluid reservoir, some of it is bound to find its way into the ABS unit. The debris can build up inside the tiny passageways of the hydraulic unit, eventually plugging them closed. Of course, this could render your ABS nonfunctional and have dangerous implications for your car’s braking performance.

Poor braking performance

When you don’t change your brake fluid, your car’s braking capabilities can degrade significantly. In most cases, you can tell simply by pressing the brake pedal—if it feels incredibly spongy, then there’s a good chance your brake fluid has nearly given up. You might notice that it takes more effort and time to bring your vehicle to a stop. In some cases, you may even have to pump your brakes to build up enough pressure for a safe stop.

How to change your brake fluid

Changing the brake fluid in your car is a fairly straightforward operation. If you have an opportunity to use a pressure bleeder, that would be the best option. A pressure bleeder hooks up to the top of the master cylinder and pumps new pressurized brake fluid to each wheel. You would then open the bleeder valves on each wheel and let the brake fluid come out until it’s clean. It’s that simple.

A few things to consider before you start:

  • Change the brake fluid with the same type of fluid that’s currently in the master cylinder. You’ll see the type of brake fluid needed on the master cylinder cap.
  • Although you will usually do this every four to five years, make sure to consult with your owner’s or service manual for your particular car.
  • A buddy or helper will make it go quickly, although you can do it yourself.
  • If possible, jack your car up and lower it onto jack stands so you can easily access all four wheels.
  • Wear gloves.

If a pressure bleeder isn’t an option, here’s a 10-step plan on how to change your brake fluid.

  1. Remove the wheels. Remove all four wheels to expose the brakes.
  2. Clean the master cylinder exterior. This is an excellent time to clean the outside of the master cylinder with brake cleaner so you can see the fluid inside the reservoir.
  3. Open the master cylinder cap. Once cleaned, open the master cylinder cap and use a turkey baster to transfer the liquid in the cylinder into an empty plastic jug or container. You don’t need to get out every last drop—just the majority of it. Throw the turkey baster away after you use it.
  4. Fill the master cylinder with fresh fluid. You could stop after doing this, and the brake fluid is “technically changed” since the majority of it is in the master cylinder. However, there are more steps if you want to change the fluid completely.
  5. Bleed the brakes. To bleed the brakes, you will do so in this order: left front, right rear, right front, left rear. The left front wheel is closest to the master cylinder, and you’ll get the most fluid through this wheel. If you were to start at the back, you would run all the dirty fluid through the whole system.
  6. Hook up your bleed bottle. Take out the little rubber plug on the caliper bleed screw and get a bleed bottle. For a bleed bottle, you can use a 20-ounce soda bottle with aquarium tubing inserted through a hole drilled in the top. Also, you’ll want a tight fit on the bleeder screw. The tubing should be long enough that it coils into the bottom of the bottle where the fluid is. This keeps air from being pulled back into the brake system and sucking back the old brake fluid. You can also buy a bleed bottle at a parts store if you desire.
  7. Loosen the bleed screw. Place the tubing on the bleeder screw and loosen the bleeder screw a bit. On many cars, you’ll use an 8mm wrench to loosen the screw.
  8. Pump the brakes. Open the door to your car and pump the brake pedal until you see clean brake fluid in the tube, which could take around eight pumps. Tighten the bleeder valve. This will go faster if you have help.
  9. Top off with brake fluid. Top off the master cylinder again because you never want to run the master cylinder dry.
  10. Repeat. Unhook the tubing to your bleeder valve and repeat the process on the right rear, right front, and left rear.

It’s straightforward enough that a DIYer with a moderate amount of automotive knowledge may be able to handle it. But as easy as it may seem, remember that working on your brakes is best left to an automotive service technician. A brake fluid change will cost around $100, but the technician will have the experience, tools, and equipment needed to do the job quickly.


About Bumper

At Bumper, we are on a mission to bring vehicle history reports and ownership up to speed with modern times. A vehicle is one of the most expensive purchases you'll likely make, and you deserve to have access to the same tools and information the pros use to make the right decisions.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.