Brake Fluid: What is it and How Does it Work
*sings* “Under pressure! do-do-do-do-do-do-do.”
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Few automotive engineers would say there’s anything more important to a car’s safe operation than its brakes. Without them, you’re mating with that Prius’ bumper, hugging that beautiful redwood, or compacting like Steve Urkle’s accordion. Your car’s brake fluid is what actuates the important parts of the braking system and ensures you halt.
Brake fluid works similarly to how your brain fires off electrical impulses to move your muscles. An input is initiated (i.e. you hitting the brake pedal), the brake fluid rushes through the brake system to the calipers and pads, and the car comes to a halt. But how exactly does the brake fluid work? And what’s it made out of?
Never fear, The Drive’s bad bunch of wrenching ne’er-do-wells are here to help and answer all your burning questions about brake fluid, its properties, uses, and how it works.
Stop! Ok, now go!
What Is Brake Fluid?
Brake fluid is a type of hydraulic fluid that is pressurized in a vehicle’s braking system. The fluid itself can be silicone- or glycol-based in its composition and comes in a few different varieties, which we’ll explain in detail below.
What Does Brake Fluid Do?
Your car’s brakes are pretty simple. When braking pressure is applied to the brake pedal, the brake fluid’s pressurized system mechanically or electronically pushes a piston through the master cylinder, which pushes fluid to the calipers, which in turn push the brake pads, thereby clamping down onto the brake rotor. This friction then reduces the speed of the vehicle.
What Are The Different Types of Brake Fluid?
You may be surprised to learn that there isn’t just one type of brake fluid! There are three, actually. Let’s get into the key differences and similarities.
DOT3 brake fluid is the most widely used brake fluid. Straight out of the bottle, DOT3 brake fluid has a boiling point of 401 degrees Fahrenheit. With used up brake fluid, that boiling point drops to 284-degrees. It’s also HIGHLY corrosive. Handle with care. It can also absorb water through the air, which is why you shouldn’t routinely pop open your brake reservoir.
DOT3 can be mixed with DOT4 and DOT5.1.
DOT4 is starting to edge in on DOT3’s universality, with many of European automotive manufacturers favoring the chemical composition thanks to its higher boiling point (446 degrees Fahrenheit) and added chemistry to reduce its water absorption. Like DOT3, it’s also HIGHLY corrosive. Handle with care.
DOT4 can be mixed with DOT3 and DOT5.1.
Unlike DOT3 and DOT4, which are polyethylene-glycol-based, DOT5 is silicone-based and features a boiling temperature of 500 degrees Fahrenheit. DOT5 does not absorb water either, though when air becomes trapped in the system, it can bubble and make it extremely difficult to remove from the brake system. It’s not recommended for cars with ABS.
DOT5 cannot be mixed with any others.
DOT5.1 is an interesting thing because its name makes no sense. It’s not silicone-based like DOT5 but features a similar boiling point as DOT4 racing fluid. It also can be mixed with DOT3 and DOT4, though there’s no reason to do so.
The Main Parts of a Vehicle’s Brake System
Now that you’ve got a bit of knowledge under your belt on the types of brake fluid, as well as what it does and is, let’s dig into what makes up a car’s braking system.
Brake Master Cylinder
The brake master cylinder is where brake fluid is stored and pushed out of when the vehicle’s brake pedal is pressed. When the driver applies braking pressure, a piston is pushed through the master cylinder and pushes the brake fluid through the lines and to the brake calipers.
Come on, son, see above!
A brake caliper is the mechanism that applies pressure to the brake pads and rotor via the pressurized brake fluid system. A caliper can have anywhere between 1-12 pistons that press the brake pad into the rotor to curb the car’s momentum.
A brake rotor is a steel or carbon-ceramic disc connected to your car’s axle. These rotors turn in conjunction with the wheels as the car moves. The pads press against the rotor to create friction.
Brake pads come in three different flavors; non-metallic organic, semi-metallic, and ceramic. Each has its own use and longevity. To find out more about brake pad construction and how long brake pads last, click here to read The Drive’s
How Long Do Brake Pads last article that more thoroughly describes the part.
A drum brake is a type of brake system that uses shoes or pads that, unlike cars that use brake rotors, are forced outward toward the walls of a drum to reduce the vehicle’s momentum. These were commonly used by passenger cars prior to mass brake rotor adoption.
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You’ve got questions, The Drive has answers!
Q. So Can You Mix Brake Fluids?
A. Certain brake fluids can be mixed, as answered above. However, due to their price discrepancies, with the prices rising from lowest (DOT3) to highest (DOT5.1), why would you?
Q. Ok, Then What Happens If Your Brake Fluid is Low?
A. You’ll lose braking pressure and stopping power If your brake fluid is low, check your dusty old manual in your glovebox for your car’s specific brake fluid requirement.
Q. Do I Really Need to Change My Brake Fluid?
A. If your braking force has reduced or the brakes feel spongy, you may either have low brake fluid or particulate matter may have been introduced into the brake fluid. Either way, you’ll want to change it as soon as possible. Otherwise, you know, see the answer above.
Q. Can I Change My Brake Fluid Myself Then?
A. You can, and it’s not that hard! All you’ll need is an empty container, a couple wrenches, a clear piece of tubing, and new brake fluid.
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