How to Check Brake Fluid

Updated on January 24, 2019
The brake master cylinder is located at the back of the engine compartment.
The brake master cylinder is located at the back of the engine compartment. | Source

Knowing how to check brake fluid helps you maintain your car brake system functioning properly. This fluid is in charge of transferring pedal braking pressure and movement to the brake assemblies in each wheel. In other words, it's what makes all the components in the brake system work together as a unit.

To do its job properly year round, it not only needs to withstand extreme temperatures without boiling or freezing, but also needs to lubricate components and control moisture in the system.

Unfortunately, the brake fluid wears out—or gets contaminated—over time, reducing its good properties, making it necessary to replace before adversely affecting the rest of the system—and your safety.

So you need to check brake fluid level and condition.

This simple guide helps you check your brake fluid properly and fast. It tells you whether you need to add new fluid, need to replace it, or even spot a potential leak in your system.


What you Should Know About Brake Fluid

Locating the Brake Master Cylinder

Checking Brake Fluid Level

Beware of Leaking Brakes

7 Reasons Why You Should Check Brake Fluid Condition

How to Check Brake Fluid Condition

What you Should Know About Brake Fluid

  • It is hygroscopic (absorbs moisture) to protect system components.
  • Has a light yellow color.
  • Doesn't compress when squeezed—ideal for automotive brake systems.
  • Lubricates pistons in master cylinder, calipers and wheel cylinders.
  • Lubricates rubber cups in master cylinder and wheel cylinders.
  • Good tolerance to temperature extremes.
  • DOT 3 fluid has a 401F boiling point.
  • DOT 4 fluid has a 446F boiling point.
  • After two or three years in use, moisture contents rises and corrosion begins.
  • Use only brake fluid with a DOT rate recommended by your car manufacturer.

Locating the Brake Master Cylinder

The brake master cylinder works as a pump, and it's the one that pushes brake fluid into each wheel brake assembly to engage the brakes every time you depress the brake pedal. And it also holds the reservoir for the brake fluid. So you need to find this component to check the brake fluid in your car.

If you open the hood, you will find the master cylinder mounted on the brake booster. The brake booster is a large, round component located on the driver's side of the firewall—back of the engine compartment. Mounted on the booster, you'll see a small, metal cylinder (the brake master cylinder) with thin metal tubes leading from the cylinder, and a metal or plastic container mounted on top—this is the brake fluid reservoir.

Brake fluid level should reach the level mark.
Brake fluid level should reach the level mark. | Source

Checking Brake Fluid Level

Newer vehicle models (mid 1980s and later) use a translucent plastic container for the reservoir above the brake master cylinder. So you don't need to remove the reservoir's lid to check brake fluid level. On the reservoir, fluid level should reach between the MIN and MAX fill lines or at the FULL line.

Older vehicle models use a metal container for the reservoir. To check the fluid level, you need to remove the lid.

  • Before you remove the lid off the mastery cylinder reservoir, wipe dust and grease off the lid and reservoir's body using a clean rag. This will prevent contamination of the system before removing the lid.
  • On these older models, you can remove the lid off by hand or screwdriver to pry off the spring clip, or unscrew the bolt from the top.
  • With the lid off, make a visual inspection of the fluid's level.
  • If you don't see a FULL line marked inside the reservoir, check that the fluid level is at about 1/4-inch (6mm) from the top of the container.
  • To add brake fluid to the reservoir, only use the type of fluid recommended by your car manufacturer, usually DOT 3 or DOT 4. You may find this information stated on the reservoir cap itself, your car owner's manual, or the repair manual for your particular vehicle car make and model. If you don't have the service manual, buy an inexpensive aftermarket repair manual through your local auto parts store or online.

For a visual reference about how to check brake fluid level, watch the next video. And now that you've checked brake fluid level, you need to check fluid condition.

Beware of Leaking Brakes

Over time, brake pads and shoes (if equipped) wear down and thin, leaving a space gap that is filled with brake fluid from the master cylinder reservoir. This causes the fluid level in the reservoir to go down a little. So the fluid level drop should fit the normal wear pattern of your brakes. Otherwise, you may be dealing with a leak.

7 Reasons Why You Should Check Brake Fluid Condition

If you have not checked the condition of the brake fluid lately, here are 7 reasons why you should:
1. Brake fluid contains a certain amount of alcohol and absorbs moisture.
2. Moisture will corrode metal lines, caliper cylinders and wheel cylinders.
3. Brake rubber hoses and seals wear out and contaminate brake fluid.
4. Usually, the brake master cylinder becomes the first victim and begins to leak.
5. If the leaking brake fluid reaches and enters the brake booster, most likely you'll need to replace it.
6. Your car's braking power and safety suffers when you neglect brake fluid service.
7. And you'll end up with an expensive repair, usually in the thousands of dollars if you have an ABS system.
Old brake fluid can ruin your brake system.
Old brake fluid can ruin your brake system. | Source

How to Check Brake Fluid Condition

Some vehicle manufacturers recommend replacing brake fluid every two years, others recommend a five year interval. Still, some don't have a schedule for it. Unfortunately, there's not a correct brake fluid service interval that can help you prevent system contamination.

Varying climate conditions throughout the year, moisture content in the environment, condition of the brake system, and removing the lid off the brake fluid reservoir, all conspire to corrode system components.

That's why it's a good idea to check the condition of the brake fluid at least twice a year, and replace the fluid at least every two to three years.

However, just looking at the brake fluid color doesn't tell you whether the brake fluid is in good condition. Brake fluid has a clear, light yellow color, but it may darken over time. And this is not necessarily an indication of fluid deterioration. On the other hand, contamination, other than moisture, can actually darken the fluid.

The most practical and cheap way to check the condition of the fluid is with the use of brake fluid test strips. You can find these chemical test strips at your local auto parts store or buy them online for about $10.00 dollars. And the test only takes a minute.

Usually, you have to:

  1. Thoroughly clean the reservoir's lid and container with a rag.
  2. Pull out a test strip from the dispenser.
  3. Remove the lid from the brake fluid reservoir.
  4. Partially dip the test strip into the brake fluid.
  5. Replace the brake fluid reservoir lid.
  6. Wait for 30 to 60 seconds for the fluid to react with the chemicals on the test strip.
  7. Read the condition of the brake fluid by seeing the change on the surface of the test strip, and comparing your results to the sample table shown on the product's package (always read the instruction on the package).
  8. The change in color of the test strip tells you whether your brake fluid is in good condition.

With this test you avoid guessing whether it's time to replace the brake fluid. The following video shows you how fast it is to check the condition of your brake fluid using brake fluid test strips.

Knowing how to check brake fluid is one of the most simple maintenance tasks a car owner can do to save hundreds of dollars in repairs. So check the level of the fluid at least once a month or every time you have to raise the hood for whatever reason—but avoid removing the reservoir lid unnecessarily to avoid rapid moisture contamination. And check brake fluid condition once or twice a year. This simple checks will help you prevent brake performance problems as well caused by corrosion from old, contaminated fluid. And replace the fluid every three years or sooner, if you live in a high-wet, humid climate area. You'll increase the system life service and reduce costly repairs and bad brake performance.

How often do you change brake fluid on your car?

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


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      4 years ago

      How reliable are brake fluid tester pens (which measure fluid conductivity and then convert that to boiling point), and which one (test pen or test strip) is better? Cheers!


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