Brake fluid reservoir

Brake Fluid Change: Why, When And How To Do It

Automotive technology never ceases to improve and braking systems are no different. However, no matter how technologically-advanced brake systems are now, they still all rely on brake fluid to do their job, no matter the brand or model. With that being said, it’s quite easy to understand why it’s important to keep an eye on the brake fluid level in your vehicle as well as to maintain it when needed. Luckily, changing brake fluid at home is an easy DIY job, it doesn’t take that long and it’s also relatively cheap. To help you change the brake fluid in your car and save money at the same time, we’ve created this article explaining all you need to know to be able to inspect, maintain and completely flush the braking system in your car. at home. with only a couple of basic tools.


What Is Brake Fluid And What Is It Used For

First things first, let’s start by explaining what is brake fluid and why it’s so important to keep in clean and full at all times. No matter the kind of car you drive, all braking systems work based on the same basic principle: Pascal’s law.

The first point demonstrated by Pascal’s law is the fact that fluids are not appreciably compressible, as opposed to air, for example. When you inflate car tires, the air compressor takes a large volume of air, compresses it, and injects it in the tires, increasing the air pressure. When it comes to fluid, the same thing isn’t true. No matter the pressure added to a certain volume of fluid, the volume will always stay the same. It’s simply impossible to compress a fluid.

The result of this demonstration brings us to the second point. Since it can’t be compressed, when pressure is applied to an enclosed fluid, the pressure is transmitted equally in all directions. In practice, when the driver depresses the brake pedal, a plunger pushes the brake fluid contained in the master cylinder through the brake line, redirecting the pressure applied equally to each wheel, allowing them to brake uniformly.

If the driver suddenly presses harder on the pedal, the pressure also increases equally to each wheel, preventing having wheels braking harder than others, which could cause a loss of control.

Types of Brake Fluid

Brake fluid can either be a natural or synthetic product. There are three primary types of brake fluid on the market today. Some types of brake fluid are compatible with others, but care must be used to prevent contamination when changing brake fluid.

The three main types of brake fluid are DOT 3, DOT 4, and DOT 5. There is also a type of brake fluid becoming more popular these days; DOT 5.1.

DOT 3, 4, and 5.1 are all glycol ether-based. All three are compatible with one another and should not cause problems with the brake system. Vehicles that specify DOT 3 can use any of the other types, but if the vehicle specifies DOT 4 or 5.1, do not use DOT 3. As a rule, you can always upgrade the stock fluid in your car but you can’t downgrade it. DOT 4 and 5.1 have a higher boiling point than DOT 3, so if the vehicle requires the use of these fluids, using DOT 3 will cause rapid brake fade, boiling of the brake fluid, and loss of brake pressure.

Synthetic Brake Fluid

Dot 5 fluid is in a category of its own and cannot be mixed with any other. Only use it if your car is already using this type from the factory. It has a high boiling point and does not absorb water like glycol ether-based fluids. DOT 5 is not compatible with any other type of brake fluid. Combining non-compatible brake fluids can cause sudden brake failure and loss of brake pressure.

How to Check Your Brake Fluid

Checking the Level

The brake fluid in your car is contained in a small plastic reservoir on top of the master cylinder. On the side of the reservoir, there will be a gauge with 2 marks: FULL (F) and LOW (L), Make sure the level is between the lines. If you have a hard time seeing the line, try wiping the reservoir with a shop rag and put a shop light on the opposite side. The light shining true will make it easier to see the level.

If the level is a bit low but still between the lines, it’s totally normal. As your brake pads and shoes wear out, the caliper pistons will come out more, which will cause the level to go down slightly. At some point in the future, you’ll replace your brake pads, and retracting the piston will bring the fluid level back to FULL.

If the level is below the LOW mark, there’s definitely something wrong here. A hydraulic brake system is a closed system that should never lose fluid. If the level of brake fluid is significantly low, or if the master cylinder keeps draining, the system is leaking somewhere. It is vital to correct any brake system leaks before taking the car back on the road. Leaks can cause sudden brake failure and you risk seeing your brake pedal go down to the floor at any time.

Auto mechanic inspecting brake fluid

Checking the Color

The color of the brake fluid says a lot about the condition of the brake system. Brake fluid should be clear and free of foam or bubbles. Brown or black brake fluid indicates old, burned brake fluid or contamination from dirt. Dirty brake fluid does not work as well as clean fluid and can cause inconsistent braking performance.

Also, foam and bubbles in the brake fluid indicate the presence of water or air in the brake system and both will significantly reduce the performance of a braking system. Since air is easily compressible, it will cause your brake pedal to go down without pushing on the fluid in the lines, often leading to a complete brake system failure.

Ok, but what’s with water? It’s a fluid right? While this is absolutely true, the problem with water is that, as you brake, the temperature of the fluid will quickly increase. As the temperature rises, water will start to boil, which will, in turn, also create air bubbles in the system, causing the same problems as air.

When to Change Brake Fluid

It is good practice to periodically replace brake fluid based on the manufacturer’s recommended intervals. However, the lifespan of brake fluid depends largely on how the car is driven. Cars that see lots of miles will need more frequent fluid changes. A four to five-year change interval is recommended by most manufacturers. If the brake fluid is contaminated with water or other substances, it should be changed before the car is back on the road.

It’s also a good idea to replace the brake fluid when doing significant repairs, like replacing the master cylinder. Changing the brake fluid is easy at that time since the system will need to be bled to remove air anyway.

These types of brake fluid are hygroscopic, meaning that they absorb water. Each time you take the master cylinder cap off, a small amount of water contained in the air will be absorbed, reducing the overall lifespan of the fluid.

How Much Does a Brake Flush Cost?

Luckily, brake fluid is quite inexpensive. Depending on where you shop, the type of fluid you need, and the container size you choose, you’ll pay anywhere from 2-3$ for a small 12oz of DOT3 fluid to 20-30$ for a gallon of DOT4. If you buy OEM brake fluid instead of an aftermarket brand, it will be a little more expensive but even then, the difference is negligible.

Most auto repair shops will also charge around an hour of labor to replace the brake fluid in your car. If you are lucky, some repair shop even has a flat rate deal for brake flush.

Obviously, the easiest way to save money on your next brake flush is to do the job yourself. More on that below!

How to Change Brake Fluid in Almost Any Car

If your brake fluid needs replacement, any average DIYer can easily flush and replace it at home with only a couple of tools and some basic auto mechanics notions. This is a simple job but it usually requires two people. Nonetheless, there are tools that professional technicians use to be able to do it by themselves. We’ll talk more about that later.

Flushing and refilling the brake system usually takes about an hour on more recent vehicles but it can easily take over two hours on older cars with rusty brake bleeders,

Auto mechanics draining brake fluid

Step 1 – Draining the Brake Fluid

Start by lifting the vehicle and place it on jack stands. You can sometimes perform a brake fluid flush with wheels on but this process is often easier and a lot less messy with the wheels removed from the car. To drain the fluid, you’ll need to open the 4 bleed screws located behind the brakes. Disc brakes will have a bleed screw on the caliper. Drum brake bleed screws, on the other hand, will be behind the wheel cylinder.

Remove the master cylinder’s cap, take your new brake fluid bottle, open it and turn it upside down into the master cylinder. This will prevent the master cylinder from sucking up some air as it’s emptied.

Place a short length of clear rubber hose into a transparent container and attach the end of the hose to the bleed screw on the brake farthest from the master cylinder. This is usually the rear, passenger-side (RR) wheel in American cars. Use a wrench to loosen the bleeder. Be very careful as bleeders tend to seize and they are super easy to break. A small amount of brake fluid will start to come out. Now, have an assistant press the brake pedal slowly. Tighten the screw once the brake pedal reaches the floor. Repeat the process until the fluid coming out of the hose becomes clear. Do the same for each wheel, always working the farthest one from the master cylinder.

If you don’t have an assistant, you can always use a vacuum pump to suck out the old brake fluid.

Different types of bleed screws
Different types of bleed screws

Step 2 – Flushing the Brake System (optional)

There are two ways to flush a brake system: the easy way and the hard way. The easy way is to just flush each wheel until the fluid becomes clear as explained above. However, some people want to do an even more thorough job. In that case, you could decide not to flip the bottle of new fluid into the master and just empty it completely first. Then, fill it with denatured alcohol. Using the same procedure as for draining the brakes, force the alcohol through the system. Continue adding alcohol and flushing it through the brake lines until it comes out perfectly clean. Use compressed air to blow through the brake lines to remove and dry the alcohol in the lines.

The main problem with this added step is that the master cylinder is now dry and will need to be bled. Bleeding a master cylinder is a much more complicated procedure than simply bleeding brake lines. Furthermore, unless you drive a really old car and the brake fluid hasn’t been replaced in 20 years, it’s probably not necessary anyway. I’ve been working in various car dealers for more than 15 years now and I’ve never even seen someone do that before.

Moving on!

Step 3 – Refill and Bleed the Brake System

Once the brake system has been flushed completely, it’s time bleed it. Make sure all bleeder screws are closed, then fill the master cylinder with clean brake fluid of the correct type from a new, sealed container up to the FULL line. Avoid using brake fluid that has been opened; there is a good chance it has absorbed water and will not perform well. If you aren’t sure which type of fluid to use in your vehicle, take a look in your car’s repair manual for the right specifications.

Bleeding Procedure

The purpose of bleeding the brake system is to remove air from the brake lines. As mentioned before, this step is a lot easier to do with an assistant but you can always do it with a vacuum pump. First, put a small amount of clean brake fluid in a clean container. Place one end of the rubber hose on the bleeder and the other end of the hose in the container. Ensure the open end stays below the level of brake fluid at all times.

Have your assistant pump the brakes in slow, medium strokes. Avoid pressing the pedal all the way to the floor. Doing so may damage the master cylinder. After three or four strokes, have the assistant hold the brake pedal down. Now, open the bleeder screw, keeping the hose below the fluid level in the glass jar. You should see bubbles of air coming out. Once the bubbles stop, tighten the screw and have the assistant pump the brakes 4-5 times again.

Check the level in the master cylinder and add some if needed. If the fluid drops below the LOW mark, the bleeding process will have to be started over. Continue this procedure at each wheel until no more air comes out. Make sure the car has a firm brake pedal. This may take several tries, so keep at it until all the air is gone from the brake lines. Keep checking the master cylinder level throughout the process. Once you have good pedal pressure, you are done changing the brake fluid.

Mechanic using a vacuum pump to bleed brakes

Another Alternative to an Assistant

A few products on the market today can help simplify and speed up changing brake fluid. One device is a type of one-way check valve used in place of the factory bleeder screw. With these in place, you can open up all four brakes and simply depress the pedal until all the air is gone.

Word of Warning

Many cars equipped with ABS systems need the help of an OBD2 scan tool to bleed the air from the hydraulic block. If it’s your case, make sure to find and follow the manufacturer’s recommended procedure or you could be doing more harm than good here.

In Brief…

Maintaining your braking system regularly is your best bet to prevent future expensive repairs. Fortunately, all you have to do is to keep an eye on the fluid level every time you change your oil and replace it once every couple of years. With a little practice, you should be able to replace your brake fluid in about an hour and save a lot of money when compared to having it done at an auto repair shop. And when in doubt, make sure to get your hand on your car’s repair manual so you can follow the specific replacement procedure for your vehicle.

You could also get replacement brake fluid, pads and rotors here. 

Good Luck!

About Derek F

Derek grew up in Southern California and started working on cars when he was a child. He learned from his father and grandfather how to make basic repairs and maintain cars correctly. Derek rebuilt his first engine at 15 years-old, beginning an automotive career that took him to many interesting jobs. Derek has worked as an automotive detailer, managed parts warehouses and auto parts stores, and worked as a mechanic for several years doing brake and suspension work. While pursuing a bachelor’s degree in government journalism, Derek worked for an auto museum where he started to write about cars. Today, Derek uses his expertise gained from many years of practical experience to help educate DIYers and share interesting knowledge about various types of automotive repair and service. Writing about cars helps fund his numerous classic car restoration and customization projects.


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    Interesting Information for the automotive DIYr

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