Changing car brakes, one car at a time…

In theory...
Imagine how much force you need to stop a fast-moving car. Simply pressing with your foot would not generate enough force to apply all four brakes hard enough to bring you quickly to a stop. That's why brakes use hydraulics: a system of fluid-filled pipes that can multiply force and transmit it easily from one place to another.
When you press on the brake pedal, your foot moves a lever that forces a piston into a long, narrow cylinder filled with hydraulic fluid. As the piston plunges into the cylinder, it squirts hydraulic fluid out through a long and narrow pipe at the end (much like squirting a syringe). The narrow pipe feeds into much wider cylinders positioned next to the car's four brakes. Because the cylinders near the brakes are much wider than the one near the brake pedal, the force you originally applied is multiplied greatly, clamping the brakes hard to the wheels.
In practice...

  1. Your foot pushes on the brake pedal.
  2. As the pedal moves down, it pushes a class 2 lever (a kind of simple machine), increasing your pushing force.
  3. The lever pushes a piston (blue) into a narrow cylinder filled with hydraulic brake fluid (red). As the piston moves into the cylinder, it squeezes hydraulic fluid out of the end (like a bicycle pump squeezes out air).
  4. The brake fluid squirts down a long, thin pipe until it reaches another cylinder at the wheel, which is much wider.
  5. When the fluid enters the cylinder, it pushes the piston in the wider cylinder (blue) with greatly increased force.
  6. The piston pushes the brake pad (green) toward the brake disc (gray).
  7. When the brake pad touches the brake disc, friction between the two generates heat (red cloud).
  8. The friction slows down the outer wheel and tire, stopping the car.
This shows the basic principle of a hydraulic braking system; in practice, there's a little bit more to it. The brake pedal actually operates four separate hydraulic lines running to all four wheels (we're just showing one wheel here for simplicity). Instead of a single cylinder, there's usually one main cylinder (sometimes called the master cylinder), operated by your foot and the brake pedal, and then one secondary cylinder (or slave cylinder) on each wheel. By making the main cylinder smaller than the secondary cylinders, we can amplify the braking force that the driver applies. Finally, for added safety, hydraulic brakes typically have two separate hydraulic circuits in case one of them fails.