Brake fluid increases the force created by pressing the brake pedal and transferring it to your wheels’ braking mechanisms, allowing you to stop your vehicle.

Written by Andrew Biro

Reviewed by Melanie Reiff

Updated on Jan 29, 2024

Table of Contents

What does brake fluid do?
Why is brake fluid important?
Elements of good brake fluid
Types of brake fluid
When does brake fluid need to be changed?
Signs of low brake fluid
How to replace brake fluid
How to find affordable car insurance

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Brake fluid is a hydraulic fluid that increases the force created when you press down on the brake pedal and transfers it to your wheels’ braking mechanisms. It has a high boiling point to withstand heavy friction, absorbs water to prevent corrosion in the brake lines, and has a low viscosity (thickness).

Knowing what brake fluid is and how it works is an important part of being a responsible car owner—it can potentially save you from suffering total brake failure. That’s why

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What does brake fluid do?

Brake fluid is what actually allows you to stop your vehicle when you press down on the brake pedal. Any vehicle that uses a hydraulic brake system—which is most cars—uses brake fluid in its braking system and cannot function without it.

Brake fluid works by amplifying (increasing) the force created when you push down on the brake pedal and transferring it to each wheel’s braking mechanism so that you can quickly stop when traveling at high speeds.

Why is brake fluid important?

Without brake fluid, your car’s brakes won’t work properly—and if your brakes don’t work properly, you can’t reliably stop your vehicle. This increases the risk you pose to yourself and other drivers.

Brake fluid is also important because it helps protect the metals used inside other brake elements, like the wheel cylinders, brake cylinders, calipers, and more. It does this by absorbing water that would otherwise lead to corrosion and boiling damage.

Elements of good brake fluid

In order for brake fluid to work properly, it must meet three requirements—the correct viscosity, a high boiling point, and resistance to corrosion.

Correct viscosity

One of the most important characteristics of good brake fluid is that it has the correct viscosity or thickness, especially when operating in low temperatures.

If your brake fluid is too thick—in which case it would have high viscosity—it will upset the hydraulic pressure needed to activate and employ the brakes.

High boiling point

When your brake pads are in use, they generate a large amount of heat, which means brake fluid must be able to withstand extreme temperatures and have a high boiling point to avoid being vaporized. This is an extreme condition that would render your brakes inoperable!

Resistant to corrosion

To avoid damaging the brake lines, (most) brake fluid is formulated to be hygroscopic, or able to absorb moisture.

This characteristic and certain additives allow the brake fluid to prevent corrosion and rust in your car’s braking system.

Brake system flush

Types of brake fluid

With that being said, not all brake fluids are the same. There are four different types of brake fluid that you should be aware of, all of which are regulated by the Department of Transportation (DOT): DOT 3, DOT 4, DOT 5, and DOT 5.1.

The table below helps explain their differences:

Type	Dry Boiling Point	Wet Boiling Point	Composition
DOT 3	401°F	284°F	Glycol-Ether
DOT 4	446°F	311°F	Glycol-Ether & Borate-Ether
DOT 5	500°F	356°F	Silicone
DOT 5.1	500°F	356°F	Glycol-Ether & Borate-Ether

DOT 3, DOT 4, and DOT 5.1 brake fluids are similar in that they are all glycol-based and absorb water, whereas DOT 5 is a silicone-based brake fluid that does not absorb water.

If you don’t know which brake fluid your vehicle uses, check your owner’s manual for the manufacturer’s recommendation. Using the wrong brake fluid can be catastrophic, and you should never mix two different types.

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When does brake fluid need to be changed?

Most vehicle manufacturers suggest that you change your brake fluid once every two years or 20,000 miles, whichever comes first, but there is no one-size-fits-all interval that can be applied to all cars.

This is largely because your driving habits influence how often you need to change or flush out your brake fluid.

Of course, you’ll also need to change your brake fluid if it starts to go bad, starts leaking, or gets too low. For more information about the signs and symptoms of low brake fluid, check the section below.

MORE:

7 common reasons your brakes are grinding

Signs of low brake fluid

To avoid putting yourself in a potentially dangerous situation, it’s a good idea to familiarize yourself with the signs of low brake fluid, four of which we’ve outlined below:

Brake pedal is soft

If your brake fluid is low, depressing the brakes may feel soft or spongy. This happens when moisture accumulates in the vehicle’s brake system and begins to boil in the intense heat produced during braking.

This also causes pressure to rise in the brake lines. Unlike brake fluid, air cannot be compressed, so it pushes the brake pedal further and impedes your ability to stop.

Fluid puddle under the car

If you find a puddle of brake fluid underneath your car—which you should be able to identify by its yellowish-brown color and strong fish-oil smell—there is a

brake fluid leak

somewhere in your brake system. In this case, you’re definitely going to have less brake fluid than you need.

ABS warning light is on

You might need to add more brake fluid if the

anti-lock braking system (ABS) warning light

pops up on your dashboard.

Most modern vehicles use sensors in the brake system to help monitor the brakes and alert the driver to any potential issues, and low oil is something that can trigger your ABS light.

Brake pads are old

Old brake pads can be both a sign and a cause for low brake fluid. Worn-down brake pads mean the brake caliper piston is forced to travel further away from the housing before contacting the pads.

When this happens, more brake fluid is needed and must subsequently travel further, which can make it run out more quickly.

MORE:

How to tell if your brake pads and shoes are worn out

How to replace brake fluid

Most of the time, you can’t do a complete brake fluid flush yourself, but you can replace a fair amount of the old fluid with new fluid, which is the next best thing.

As long as you have the proper tools and know-how, checking and replacing brake fluid isn’t a difficult task, but it may require you to get your hands dirty. If that’s not a deal-breaker, start by gathering the following:

Carjack and jack stands

Fluid pump or turkey baster

Empty container

Clean rags

Small rubber hose

A second person

Once you have the necessary tools—and the correct brake fluid for your type of car—follow the steps below:

Use the jack to lift your vehicle off the ground and set it on jack stands. Remove all four wheels (not necessary, but makes the process much easier).

Remove the cap from the brake master cylinder and use the turkey baster to remove as much of the old brake fluid as possible. Dispose of it in a sealable container.

Wipe out the brake fluid reservoir with a clean rag.

Fill the reservoir up to the MAX line with new brake fluid.

Go to the brake caliper furthest from the master cylinder, find the bleed valve—it should be on the back—and clean it off.

Attach the small length of rubber hose to the valve and submerge the other end in the container of old brake fluid.

Stick a block under the brake pedal and have your helper pump the brakes three times. Then have them hold the pedal down. As the pedal is being held down, loosen the bleed valve and allow the old fluid to drain out. Repeat the pump/hold process until new, clean fluid comes out of the bleed valve.

Periodically check your car’s brake fluid level and add more fluid to ensure it does not run dry. This will allow air into the brake lines, and you’ll have to start the process over.

Repeat this process for each wheel. Once you’re done with all four, put the wheels back on and raise your vehicle up and off the jack stands before lowering it to the ground once more.

As long as you follow these instructions carefully, you should have no issues replacing your brake fluid at home. Over the next few days, check beneath your vehicle for fluid leaks and address them if necessary.

MORE: How to check and add brake fluid

How to find affordable car insurance

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What happens if brake fluid is low?

If your brake fluid gets too low, air can make its way into your brake lines, leading to what is often called “soft” brakes, or brakes that feel spongy when the pedal is depressed.

This can make it much more difficult to stop your vehicle, putting you and other drivers at higher risk for an accident or collision.

Can I drive my car if it needs brake fluid?

While it is possible to drive your car with low brake fluid, it is not recommended and a safety hazard. If at all possible, only drive your vehicle to the nearest automotive repair shop.

What is brake fluid made of?

Most brake fluids—DOT 3, DOT 4, and DOT 5.1—are made using glycol-ethers, a group of solvents that can withstand very high temperatures due to their high boiling point.

DOT 5 is the only non-glycol-based brake fluid and instead uses a silicone base, which negates its ability to absorb water.

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