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Brake Fluid Explained and Simplified

 

Brake fluid reservoir of a car showing the max fill line

Brake fluid is a hydraulic fluid that is part of your car's braking system. Hydraulic brakes harness the physics of fluid dynamics (helped along by manifold vacuum in most "power brake" systems) to apply more pressure to the brake pads or shoes than would be possible using muscle power alone.

The brake fluid is stored in the brake fluid reservoir, which is near (or sometimes part of) the master cylinder assembly and the brake booster.

Of all the fluid systems in your car, the brake system is the most sensitive to debris. Always clean the brake fluid filler cap and surrounding area before removing it. Even slight contamination of the brake fluid with debris can have fatal consequences.

In addition to its hydraulic function, brake fluid also lubricates and conditions the moving parts and seals of the brake system and protects the metal parts from corrosion. Most manual-transmission cars also use brake fluid as clutch fluid, using either the same reservoir as the one used for the brakes or a separate reservoir.

 

Types of Brake Fluid

There are five types of brake fluid, but only three are in common use on modern cars. DOT3 and DOT4 brake fluid are glycol-based, and DOT5 is silicone-based. The filler cap will have markings to tell you which kind of fluid your car's brake system uses.

You must use the correct brake fluid for your vehicle. Using the wrong fluid can damage your brake system and cause complete brake system failure. Here are the basic rules as of this writing (in November of 2019):

Brake fluid is highly irritating to the skin and eyes. Always wear chemical-resistant gloves and eye protection when handling it. It also will melt most paints right off a car, so don't spill it. It's pretty nasty stuff.

Brake Fluid Maintenance

Most of the time, you really won't need to (and shouldn't') do anything to the brake fluid except check its level, which usually can be done without removing the cap. Unlike motor oil, you usually won't need to (and generally shouldn't) add fluid to a brake system. Here's why.

As mentioned earlier, brake fluid is a hydraulic fluid. It applies pressure to the brake pistons to force the brake pads or (or brake shoes) against the brake rotors (or brake drums) to slow and stop the car. As the brake pads or shoes wear down, it's normal for the brake fluid level to drop because it takes more fluid to maintain the brake clearances. As long as the brake fluid level is above the "MIN" line on the reservoir housing, there is no need to add fluid. If you do, the reservoir will probably overflow when you have the brakes replaced.

What to Do if the Brake Fluid is Low

If your brake fluid does drop below the "MIN" line, then either your car is in need of a brake job, or you have a leak somewhere in the system. Either way, the problem needs to be checked out by a mechanic.

If the brake fluid level dropped gradually and is only slightly below the "MIN" line, then you probably need a brake job. Assuming that there's no squealing, grinding, loss of braking ability, brake pedal fading, or other signs of a critical brake problem, you can add just enough fluid to bring the level to the "MIN" line and drive the car to a nearby mechanic's shop.

If the brake fluid level is far below the "MIN" line, is not visible at all, or dropped rapidly, you probably have a leak in the system and the car probably isn't safe to be driven. You should have the car towed to a mechanic.

What Does it Mean to Bleed the Brakes?

Mechanic bleeding the brakes of a car using a pistol grip vacuum bleeder with the tire and wheel of the car removed

"Bleeding the brakes" means removing enough fluid to purge any air bubbles from the lines. It's always necessary if the part of the brake system containing the fluid has to be opened for any reason (for example, to replace a brake piston).

Most mechanics also bleed the brakes when the brake shoes or pads are changed. It's not strictly necessary because the hydraulic system hasn't been opened, but it's generally considered good practice.

Professional shops and hardcore DIY car mechanics usually use a pressure brake bleeder that provides brake fluid under mild pressure to the brake reservoir, which slightly pressurizes the brake lines. The mechanic will then attach a piece of clear brake bleeder tubing (aquarium tubing works) to each brake bleeder valve or screw, stick the other end of the hose in a clear jar, and open each bleeder valve or screw until the fluid is free of bubbles.

Another way to bleed brakes uses the opposite approach. A vacuum brake bleeder is attached to each bleeder valve or screw to pull the fluid out until there are no more bubbles. When brakes are bled this way, it's important to make sure that the fluid level in the reservoir doesn't drop too low, lest more air be sucked into the system than is removed from the system.

Occasionally, DIY mechanics bleed the brakes using nothing more than a piece of clear hose, a clear jar, and a helper. They attach one end of the hose to each brake bleeder valve or screw, and the other end into a clear jar of brake fluid. The mechanic opens the bleeder while the helper slowly applies the brake to pressurize the system, and when the bubbles stop, the mechanic closes the bleeder. As with the vacuum bleeder system, it's important to monitor the fluid level in the reservoir so it doesn't drop too low and start drawing air into the system.

Replacing the Brake Fluid

Brake fluid doesn't last forever. Over time, DOT3 and DOT4 fluids attract water, which lowers the fluid's boiling point. Heat also takes its toll on the fluid, as do chemical changes that occur over time. At some point, the fluid will have to be changed.

You should consult your car's owner's manual to determine the proper brake fluid change interval for your car. If none is specified, then my personal rule of thumb is that it should be changed as part of every other brake replacement job or every three years, whichever comes first.

Brake fluid is replaced using the same procedures as bleeding the brakes. The only difference is that more fluid is removed. The mechanic can usually tell when he or she is getting to the new fluid either by observing the fluid in the clear tubing (new fluid tends to be cleaner and clearer than old fluid), or by looking in the jar or canister to see how much fluid has been removed.