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 six types of brake fluid, but only four are in common use on modern cars:

DOT 3 brake fluid is glycol-based and is used in many late-model brake and clutch applications. It has a minimum dry boiling point of 205°C (401°F) and a minimum wet (degraded) boiling point of 140°C (284°F).
DOT 4 brake fluid is glycol and borate-based and is used in many late-model brake and clutch applications. It has a minimum dry boiling point of 250°C (446°F) and a minimum wet (degraded) boiling point of 155°C (311°F). It is backward-compatible and can be mixed with DOT 3, but must be changed more frequently because it is more hygroscopic (attracts more water). DOT 4 brake fluid should be replaced every two years, even if the car is lightly-driven or not driven at all.
DOT 5.1 brake fluid is actually in the DOT 3 / DOT 4 line. It is borate and glycol-based and is used in many late-model brake and clutch applications. It has a minimum dry boiling point of 260°C (500°F) and a minimum wet (degraded) boiling point of 180°C (356°F). It is backward-compatible and can be mixed with DOT 3 and DOT 4, but is even more hygroscopic than DOT 4. DOT 5.1 brake fluid should be replaced every year, even if the car is lightly-driven or not driven at all. DOT 5.1 brake fluid is not compatible with DOT 5 brake fluid, and the two cannot be mixed.
DOT 5 brake fluid is silicone-based and is used in very recent-model cars. It is not compatible and cannot be mixed with any other kind of brake fluid, and should only be used in cars that specify it. DOT 5 brake fluid has a minimum dry boiling point of 260°C (500°F) and a minimum wet (degraded) boiling point of 180°C (356°F). It is not very hygroscopic and only needs be changed in accordance with the vehicle manufacturer's specification or when it has been contaminated. Manufacturer-specified change intervals for DOT 5 typically are typically three to five years.

Brake Fluid Compatibility

You must use the correct brake fluid or a compatible fluid for your vehicle. Using the wrong fluid can damage your brake system and cause complete brake system failure. Here's the nutshell version of brake fluid compatibility and miscibility as of this revision (in May of 2022):

If your car calls for DOT 3, then you can use DOT 3, DOT 4, or DOT 5.1 (but not DOT 5).
If your car calls for DOT 4 , then you can use DOT 4 or DOT 5.1 (but not DOT 5).
If your car calls for DOT 5, then you must use DOT 5
Never mix DOT 3, DOT 4, or DOT 5.1 brake fluid with DOT 5. They are not compatible.

Does it Pay to Upgrade from DOT 3 to DOT 4 or DOT 5.1 Brake Fluid?

If your car's manufacturer specified DOT 3 brake fluid, upgrading to DOT 4 or DOT 5.1 may seem like a no-brainer. The same goes for upgrading from DOT 4 to DOT 5.1. It just seems like common sense to use the "better" brake fluid.

Unless you drive in severe conditions, however, upgrading the brake fluid type may not be worth the higher cost and the more-frequent brake fluid changes. Remember that the manufacturer chose the fluid specification based on a wide range of vehicle operations; so whatever the manufacturer specified should be adequate for most drivers.

Upgrading to a brake fluid with higher heat tolerance makes sense if any of the following conditions apply to your driving:

You race or pull a trailer, even if only occasionally.
You drive in stop-and-go traffic on a regular basis.
You carry more than one average-sized passenger on a regular basis. People have weight, and increased weight requires more braking force to stop the vehicle.
You frequently carry heavy stuff in your vehicle.
You drive in an area with mountainous or hilly terrain on a regular basis.

Otherwise, whatever brake fluid your car's manufacturer specified will probably be fine.

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.

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.

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, DOT 3, DOT 4, and DOT 5.1 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.