A brake caliper turns the pressure of your foot into a clamp. Use the buttons to look inside it and follow the brake fluid as it drives a piston, squeezes the pads onto the spinning disc, and how a hidden rubber seal lets everything spring back when you lift off.
A disc brake caliper is the clamp that squeezes a spinning brake disc to slow a wheel. When you press the brake pedal, hydraulic fluid is pushed into the caliper and drives a piston out of its bore. That piston presses a brake pad against the disc, while the caliper body slides and brings the second pad in from the other side, so the disc is gripped from both faces at once.
The friction between the pads and the disc is what actually slows the car, turning the wheel's motion into heat. A disc brake is the modern standard on cars because it stays cooler and grips more consistently than a drum brake, and its working parts are simple: a bore, a piston, two pads and a seal.
From the outside a caliper is just a metal block bolted over the disc. Its piston, its bore and the square-cut seal that both seals and retracts the piston are all hidden inside, so opening it up shows how a light push on the pedal becomes a powerful clamp.
One idea holds it together: hydraulic pressure is transmitted, not created, and the piston turns that pressure into movement. The square-cut seal is the quiet hero — it keeps the fluid in, and its own deformation is what lets the brakes release cleanly when you lift your foot.
Pressing the pedal pushes brake fluid through the lines and into the caliper. Because the fluid cannot really be squeezed, the pressure it carries is the same at the master cylinder and at the caliper, ready to act on the piston.
The pressurised fluid fills the space behind the piston. With nowhere else to go, it shoves the piston out along the bore, straight towards the spinning disc.
A rubber square-cut seal sits in a groove around the piston. As the piston slides out it drags the seal slightly with it, deflecting the rubber. That deflection stores elastic energy, like a spring being wound up.
The face of the piston pushes the inner brake pad against the disc. Friction begins to bite, turning some of the wheel's motion into heat.
The caliper is mounted on guide pins so it can slide. As the piston pushes inwards, the reaction force slides the whole caliper the other way, drawing the outer pad in against the far face of the disc.
With the piston acting on one side and the sliding caliper on the other, the pads pinch the disc from both faces. The harder you press the pedal, the harder they grip, and the faster the wheel slows.
When you lift off the pedal the fluid pressure drops. The deflected square-cut seal springs back to shape and pulls the piston a small distance back with it, just enough to let the pads clear the disc so the wheel spins freely again.
Through hydraulics. Brake fluid cannot really be compressed, so the pressure you create at the pedal is carried through the fluid to the caliper, where it acts on a piston with a much larger area. That area difference multiplies the force, so a modest push on the pedal becomes a strong squeeze on the disc.
Pressurised brake fluid. When you press the pedal, fluid is forced into the caliper behind the piston, and because the fluid has nowhere else to go it drives the piston out of the bore and against the inner brake pad.
Because of the square-cut seal. As the piston moves out it deflects the rubber seal, which stores elastic energy. When the pressure is released the seal springs back to its original shape and pulls the piston a little way back, just enough to stop the pad dragging on the disc.
The caliper floats. A simple caliper has pistons on only one side and slides on guide pins. The piston presses the inner pad while the reaction force slides the whole caliper sideways, pulling the outer pad in from the other side of the disc, so both pads clamp at once.
It removes air. Air compresses easily, so any trapped in the hydraulic circuit would soak up the pedal movement and give a spongy feel. Opening the bleed screw lets that air escape while fresh fluid takes its place.
Because heat changes the friction. Hard, repeated braking can overheat the pads and the fluid. The pads grip less well, and in extreme cases the fluid can boil, creating gas that compresses like air and leaves the pedal soft. That loss of stopping power is called brake fade.