Can Brake Fluid Go Bad?

CARS.COM — Can brake fluid go bad? Indeed it can, and you might not be able to tell if it has just by looking. Most cars have see-through master cylinder reservoirs for brake fluid under the hood so owners can check at a glance to make sure it’s at the proper level. That, however, tells you nothing about the condition of the fluid.

Related: How Far Should I Go on My Brakes?

Braking systems are hydraulic and filled with fluid. Brake fluid absorbs water over time, particularly in areas with high humidity, when moisture seeps through a car's rubber hoses and seals. Water reduces the boiling point of brake fluid, and in situations that put high demands on your brakes — such as mountain driving, towing or making repeated hard stops — the fluid can become so hot that it impairs stopping ability or causes temporary loss of braking power. Gas bubbles are introduced to the brake lines and calipers when brake fluid boils, and because gas is compressible and brake fluid shouldn't be, the brake pedal travels farther, feels spongier and braking is less effective.

Checking the Brake Fluid Level

Because of this, some manufacturers recommend changing your car's brake fluid as often as every two years to prevent contamination. Others have longer intervals, and some make no recommendation about changing brake fluid. Owners should consult their vehicle's owner’s manual to see what the manufacturer recommends. If you’re unsure what to do, consult a repair shop and ask them to test your brake fluid.

One sign that your brake fluid is contaminated with water is that your brake pedal will have more travel than normal. That could also be a sign that your fluid level is low, the brake pads are worn or there’s air in your brake lines. Because the same symptom can suggest a variety of issues, having a trusted mechanic perform a thorough brake diagnosis is your best course of action. Seasoned mechanics can usually tell if fluid needs changing, as it will be dark and murky; if in doubt, ask your mechanic for more proof than an eyeball test.

You can check brake fluid levels yourself by locating the car's brake master cylinder reservoir. The reservoir that holds the brake fluid is typically located under the hood on the driver's side near the firewall. The reservoirs are often clear, but years and tens of thousands of miles of driving means you might have to take a rag and scrub engine grime and dirt off the reservoir to get an accurate look at the level, which should be near the full mark. Low brake fluid could mean a few things as mentioned above, including issues such as the brake pads being worn or there's a leak.

What Type of Brake Fluid to Use

There are various types of brake fluid: DOT 3, DOT 4 and DOT 5. DOT 4 has a higher boiling point than DOT 3 and may be a good addition for heavy use like towing and high-performance cars (if your vehicle allows DOT 4). DOT 5 has a higher boiling point than DOT 4 but is silicone-based, which means it's less susceptible to absorbing moisture like the glycol-based DOT 3 and DOT 4 brake fluids. You still need to check and use the fluid recommended for your vehicle because brake system damage can occur if you add DOT 5 fluid to a vehicle that specifies DOT 3 or 4.

Though some repair shops use maintenance or repair visits as an opportunity to sell additional services, that doesn’t mean you should dismiss a sales pitch for fresh fluid. In addition to affecting brake performance, contaminated brake fluid with water in the brake fluid can also cause corrosion over time. Down the road, you may need to replace rusted components in your brake system.  

The bottom line: When it comes to brakes, it’s better to be safe than sorry. For more on how to keep your brakes in good shape, plus a guide to the rest of your vehicle's fluids, check out the video below.

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