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How Brakes Work

Learn about brakes, or you won’t know what you’ve got ‘til it’s gone

To paraphrase the writer Gertrude Stein, “I've had brakes. I’ve had no brakes. And it's better to be rich.” When your brakes fail, I’d gladly trade riches for stopping power.

As a race driver, I’ve battled brakeless and near-brakeless cars. And just kept on keepin’ on. I wasn’t hoping for a miracle: St. Christopher doesn’t do brake jobs. Instead I imagined Kathleen Turner in Body Heat: “You're not too smart, are you? I like that in a man.”

In one 24-hour race, I drove almost an hour without touching the brake pedal. A front brake caliper had broken free from its mounts. At my insistence, the crew crimped off the broken brake line with Vice-Grips, and I reentered the race. With brakes on only three wheels and the system full of air, the car danced around like a drug-induced chipmunk—and produced no useable stopping power. So I stayed away from the pedal and coasted a lot. We won the race partially because that brakeless period allowed the crew to prepare for a quick caliper replacement. Also, we had the fastest car.

The Most Critical Component

Brakes are the most critical component on your vehicle, but here’s what most people know about brakes: Push the brake pedal and the car slows down.

Want to know more? Pushing the brake pedal creates hydraulic pressure. A vacuum booster helps create more hydraulic pressure than you could with leg power alone. The hydraulic pressure travels though tubes to each wheel. There, it pushes some really tough stuff against spinning rotors that are connected to the wheels. The resulting friction converts motion into heat. And the car slows down.

Caliper brakes on a bicycle are very similar to disc brakes on a car. Squeezing the bike’s brake lever tightens cables. This clamps material against the wheel rim, creates friction and the bike slows down. The big difference: Cable-operated brakes wouldn’t stop modern cars.

Into the late 1920s, many cars had cable- or rod-operated brakes, sometimes only on the rear wheels. Twenty mph then must have felt like 120 today. Even with four-wheel brakes, if modern cars were equipped with cable- and rod-operated systems, few beyond NFL lineman could create adequate braking force.

Hydraulic Braking

Hydraulically operated brakes radically reduce how hard you have to push on the brake pedal. That’s because hydraulics have an amazing property: A moderate amount of force applied to a small hydraulic piston becomes a huge amount of force when transmitted to a larger area. If you put 200 pounds of pressure against a piston of one square inch of surface area, hydraulic fluid will produce 4,000 pounds pressure on 20-square-inch piston down the line. And it will do so whether you have one or many 20-inch pistons in the system. (The small piston is in the master cylinder that is mechanically connected to the brake pedal. The large piston(s) are in the calipers near the wheels.)

Even the advantage offered by hydraulically operated brakes would not be enough for many drivers. That’s why virtually every car is equipped with power assist, which usually employs vacuum created by the engine. When you press on the brake pedal, the vacuum helps move it toward the floor: It’s like having that NFL lineman help with your leg presses. If the engine stops running, enough vacuum pressure remains for two or three braking applications. Even with the vacuum depleted, the brakes will still work: You’ll have to push a lot harder on the pedal—maybe use both feet—but the hydraulic system will continue to function.

Discs and Drums

To simplify things, we’ve used disc brakes for our examples. Disc brakes employ a caliper. Think of a sophisticated version of the bicycle’s brake caliper or a carpenter’s C-clamp. It surrounds about a third of the spinning brake rotor (a.k.a. disc). Moved by hydraulic pressure from the master cylinder, pistons in the caliper squeeze the friction material against the rotor.

Another type of brake system is drum brakes. These employ semi-circular “shoes” that are pressed outward against the inside of something that looks, believe it or not, like a drum. Four-wheel-drum brakes were common on American cars well into the 1970s. When used on the front axle, drum brakes don’t do a great job of providing stopping power, especially under high-speed or repeated stops, or in deep water. Today, drum brakes are effectively employed on the rear wheels of many vehicles. One reason: Front brakes do about three-quarters of the work. Another: It’s difficult to make parking brakes work on disc brakes: Cars with rear discs usually employ a small drum brake as the parking brake. Also, drum brakes are cheap to make.

Years ago, a very popular friction material for brakes was asbestos. Health concerns have reduced—but not eliminated—its use. Today’s brake pad material contains, depending on application, ceramic, carbon, iron, steel, brass, cooper, or other materials. These are bound together with either pressure and temperature or various resins.

Brake Fluid

Arguably, the single most important component of any brake system is the brake fluid. This is a very complex concoction that must operate in extreme environments, provide lubrication to the system, prevent corrosion, and more. It has to work when it’s 30-below zero. And it’s got to keep on working when emergency or repeated high-speed braking pushes fluid temperature toward 300 degrees. (On a racetrack, it may go well beyond that!)

At extreme temperatures, brake fluid can turn into vapor, as will moisture in the system. All brake fluid contains some moisture because it naturally sucks water out of the air.

Having vapor in the brake system feels as if brake fluid is squirting out of a hole in the system. The pedal mushes toward the floor. The pads don’t push hard against the rotors. The driver’s heart rate accelerates. At this point, racers give the pedal several quick pumps in an attempt to rebuild hydraulic pressure.

The U.S. Department of Transportation has set boiling-point standards for brake fluids. The most common brake fluids are DOT 3 and DOT 4. Both are mineral-based, usually poly glycol ether. DOT 4 employs another component (usually borate ester) to give it a higher boiling point (but it’s more prone to absorb water). DOT 5 brake fluids offer very high boiling points but because they are silicone based, they don’t mix with either other brake fluids or with some original-equipment brake system components. Synthetic, non-silicone brake fluids—sometimes called DOT 5.1—offer boiling points matching or exceeding DOT 5, but are fully compatible with DOT 3 and DOT 4 fluids.

If your car came with DOT 3, it’s okay to upgrade to DOT 4, though DOT 3 is just fine for the vast majority of drivers. But if your car came with DOT 4, use only that or a fluid that clearly says it will mix with DOT 4.

Level Check

Regularly check the level of brake fluid in the master cylinder: Brake wear must be offset by adding brake fluid. And dropping fluid level signals a serious problem is occurring. Also, flushing and renewing the brake fluid every two or three years can extend the life of internal brake components and improve emergency stopping performance.

If you don’t believe brakes are the most important system on your car, consider this: What’s the point of going somewhere if you can’t stop when you get there? Don’t ignore your brakes.