On a disc brake, the caliper is the component that actually does the squeezing. Press the pedal and the caliper clamps the brake pads against the spinning rotor, and that friction slows the wheel. It is a simple idea with a few important parts.
How a caliper works
When you press the brake pedal, hydraulic fluid pushes one or more pistons inside the caliper. Those pistons force the inner brake pad against the rotor. On the most common design - a floating caliper - the caliper body then slides on its guide pins and pulls the outer pad in to meet the other side of the rotor. Both pads clamp, the wheel slows, and when you release the pedal the caliper lets go.
The main parts
- Piston(s): push the pad against the rotor under hydraulic pressure.
- Piston seal: keeps fluid in and helps pull the piston back slightly when you release.
- Guide pins and boots: let a floating caliper slide freely; the boots keep out water and grit. Worn pins are a top cause of sticking - a guide pin kit renews them.
- Bleeder screw: lets trapped air out of the system when you bleed the brakes.
Signs of a bad caliper
- Pulling to one side when braking, or a wheel that gets very hot.
- Uneven pad wear from one side to the other.
- A fluid leak around the caliper.
- Dragging brakes that hurt fuel economy.
For a deeper look at sticking, see our guide on brake caliper sticking. When you service a caliper, replace the pads and hardware at the same time - and if you are not comfortable working on brakes, have a professional do it.