How to Check Brake Fluid

To check your fluid level, you'll need to let your car cool down if it's been running, then access the engine underneath your hood [source: AutoMD]. How you open the hood varies with different cars and trucks, too (again, your owner's manual can help with this). There's usually a lever or button inside the cabin that's accessible to the driver, often along the left side. Once you find and activate it, you should hear the hood "pop" open.

As a safety measure, popping the hood is only the first step. Step around to the front of the vehicle and feel under the center of the hood. You'll find a lever that you need to move or pull until you can lift the hood up the rest of the way. Be sure to secure the hood so it doesn't fall back down. Newer vehicles sometimes have a built-in system, but others will require you to lift a brace to hold up the hood.

Next, locate the brake fluid reservoir. The owner's manual should tell you the specific location for your particular vehicle, but in most vehicles you can find it on the driver's side near the firewall (the wall between your engine and the cabin).

The reservoir itself is relatively small compared to other reservoirs and has a screw cap on top. It might not say "brake fluid," but you should see various instructions on the cap, the reservoir or both. These instructions tell you to clean off the cap before opening, while the other tells you what DOT type of brake fluid to use.

Cleaning off the cap before you open it helps keep the brake fluid clean and free of contaminants (including moisture), which can make your brakes work less efficiently and even corrode the interior of your brake system [source: Ramsey]. It could even lead to a failure of your anti-lock braking system [source: Weissler]. So be safe: Take a clean rag and wipe off any loose particles from the cap.

In the next section, we'll learn about the big debate among brake fluid connoisseurs and what category of brake fluid might be right for you.

[Source: AFCO, USDOT]

If you dig deep enough into any subject, you'll find controversy. Brake fluid's big issue is whether to use glycol- or silicone-based brake fluid. They don't play well together, so knowing which one to use is critical for the performance of your brakes.

On the previous page, we learned that your vehicle's brake fluid reservoir has instructions on what type of fluid to use. Brake fluid is referred to by its DOT number and can be any of four varieties, though most passenger cars in the United States take either DOT 3 or DOT 4. The DOT part refers to the U.S. Department of Transportation, which issues detailed standards for all motor vehicle brake fluids in the country. Federal Motor Vehicle Safety Standard No. 116 details three different classifications of glycol-based brake fluids, and one silicone-based brake fluid (SBBF) [source: USDOT]. DOT 5 uses silicone instead of glycol, which is now the standard for most military vehicles. For race cars and show cars, you'll want to go with this type because, unlike glycol, it won't eat the paint off the car.

Beyond that, what's the big difference between the four categories? The answer is twofold. First, there's the boiling point. The higher the DOT number, the higher the boiling point. Remember that temperatures at the can reach up to around 800 degrees Fahrenheit (426.7 degrees Celsius). While it's important to note that these high temperatures are only where the wheels make contact with the brake pad (not in the brake fluid itself), some of that heat is being transferred into the fluid, which needs to be stable in order to function.

The second part of the answer is about water. Glycol-based brake fluid is hygroscopic, meaning it readily absorbs moisture found in the air. This is good in the sense that you can't keep moisture out, so it's designed to absorb the small amounts that reach it and still do its job. The downside of this is that the more moisture in your brake fluid, the lower its boiling point. Silicone-based fluid does not absorb water, but rather separates out the denser water and lets it settle to the bottom of your master cylinder, leaving the brake fluid to operate in its pure form and keep its boiling point high.

Next, let's take a look under the hood to see how your brake fluid is doing.

iStockphoto/Thinkstock
What a typical passenger car brake fluid reservoir looks like under the hood.

Now comes the fun part. Twist off the fluid reservoir cap, locate the dipstick and wipe it off with a clean, dry rag. Screw it back on, then off, and look at the dipstick. See where the line is between the bottom wet part and the dry part? This line should be somewhere between the "add" and "full" marks. As the brake pads wear down, the brake fluid level will drop slightly. This is natural and is not something to worry about.

Brake fluid is not something that gets consumed, so if it's reading significantly below the "add" mark, then you probably have a leak. If this is the case, get it checked immediately as the brakes are at risk of failure.

Look at the fluid to see if it's dark or clear. Dark fluid indicates significant contamination in the fluid, and you might consider getting your brakes flushed or bled. You can also check the color based on the DOT type (see table on previous page).

If your brakes feel spongy when you depress the pedal, you probably have air in your system. Because air is much more compressible than liquid, the force from your foot won't be delivered efficiently to the brake pad. This alone is a good reason to have your brake system checked.

If your brakes are working properly, and the fluid looks clear and close to the "add" mark, you should add a little brake fluid. Never fill the reservoir above the "full" mark as this could cause damage to your brake system or cause it to overflow. This is bad for two reasons: Brake fluid is extremely corrosive and toxic, and besides eating the paint off your car, it can also lead to blindness and other ailments if it contacts your skin [source: Vaphiades]. Using a latex or similarly protective glove, along with a funnel dedicated solely to this purpose, pour just enough brake fluid into the reservoir to reach slightly below the "full" mark.

Your brake fluid should last 1 to 2 years, but it's in your best interest to check it once a month to make sure the level is adequate and the fluid is clean, especially before a long road trip or if you'll be towing something [source: Ramsey].