Parts of a Brake Caliper – Disc Brake Parts Labeled

All 12 parts on the diagram

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Rotor (brake disc) The iron disc that turns with the wheel. The pads squeeze it to slow the car.
Hub The centre part the rotor and the wheel are mounted on. The wheel studs stick out of it.
Caliper The clamp that fits over the rotor's edge. It holds the pads and the piston, and does not turn.
Bleed screw A small valve at the caliper's highest point. It is opened to let trapped air out of the brake fluid.
Brake hose Brings brake fluid from the car's steel brake line to the caliper. A flexible hose, it bends as the wheel moves.
Cooling vanes In a ventilated rotor, the vanes join its two faces. They leave air passages between them, so the rotor cools faster.
Friction material (lining) The part of each brake pad that presses on the rotor. It wears away, so the pads must be replaced.
Backing plate The steel plate the friction material is fixed to. The piston or the caliper pushes on it.
Piston Fluid pressure from the brake pedal pushes it out of its bore. It presses the inner pad on the rotor. A floating caliper then slides to press the outer pad too.
Piston seal A rubber ring in a groove in the bore. It keeps the fluid in and pulls the piston back a little when the brake is let off.
Dust boot Keeps dirt and water out of the bore. A rubber cover between the piston and the caliper.
Wear indicator A small metal tab on the pad. When the lining wears thin it rubs the rotor and squeals, telling the driver the pads need replacing.

How this list was compiled: Read 2026-09-27. (1) US Army, TM 9-8000, Principles of Automotive Vehicles (1985), chapter 34, section III, Disk brake mechanisms, and figures 34-11 to 34-13, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 : the rotor or disk (solid or vented, with air passages between the friction surfaces), attached to the hub; the caliper, the stationary member; pistons with seals and dust boots in bores in the caliper; the piston seal's rolling action; brake pads (lining on a brake plate or shoe); the lining wear indicator that squeals; the floating caliper that moves laterally; fluid through steel lines and flexible high-pressure hoses; the bleed screw. (2) Wikipedia, Disc brake, https://en.wikipedia.org/wiki/Disc_brake : calipers squeeze pads against a disc or rotor; the caliper houses the pads and pistons; floating and fixed calipers; a seal or rubber protection boot; the pad's steel backing plate; fins or vanes linking the disc's two contact surfaces; a thin piece of soft metal that squeals when the pads are thin. Brake bleeding, https://en.wikipedia.org/wiki/Brake_bleeding : a bleed screw at the highest point on each caliper. Hydraulic brake, https://en.wikipedia.org/wiki/Hydraulic_brake : hydraulic brake hose from the steel line at the frame to the caliper. Licence: own work

The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work

How it works

These are the parts of a car's disc brake and its caliper. The first view is the brake with the wheel off; the second cuts a floating caliper in half across the rotor's edge.

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How it stops the car

Pressing the pedal sends brake fluid through the brake hose into the caliper. The fluid pushes the piston out of its bore, pressing the inner pad on the rotor. A floating caliper then slides the other way, pulling the outer pad on too.

What wears and what seals

Each pad is friction material on a steel backing plate; the lining wears thin, and the wear indicator squeals when it does. The piston seal keeps the fluid in and the dust boot keeps dirt out. The bleed screw lets trapped air out.

A high squeal when braking can be the wear indicator rubbing the rotor: the pads are due to be replaced.

Questions and answers

What are the main parts of a brake caliper?

The caliper body, one or more pistons with their seals and dust boots, the two brake pads it holds, and the bleed screw. Brake fluid reaches it through the brake hose.

What is the difference between a floating and a fixed caliper?

A floating caliper slides side to side and usually has one piston that pushes the inner pad. A fixed caliper does not move: it has pistons on both sides of the rotor.

Why do some rotors have vanes inside?

A ventilated rotor has two faces joined by vanes, with air passages between them, so it gets rid of heat faster than a solid rotor.

Parts of a Brake Caliper and Disc Brake