Inside every disc brake caliper is one or more pistons - the parts that actually turn hydraulic pressure into stopping force. Small but critical, the piston is where fluid pressure becomes physical clamping. Here is how it works.
How the piston works
When you press the brake pedal, brake fluid pressure enters the caliper behind the piston. That pressure pushes the piston out of its bore, and the piston presses the inner brake pad against the rotor. On a floating caliper, the reaction then slides the caliper to pull the outer pad in too - see how a caliper works. When you release the pedal, a square-cut seal around the piston relaxes and gently pulls it back just enough to release the pad.
The seal does double duty
That piston seal both keeps fluid in and provides the slight retraction that releases the brake. A dust boot around the piston keeps out water and grit. When the boot tears, moisture and dirt reach the piston and bore, leading to corrosion and sticking.
Signs of a sticking piston
- A dragging brake and a hot wheel, because the piston will not retract - see caliper sticking.
- Uneven pad wear - the inner pad on that wheel wears much faster.
- A pull to one side when braking.
- A caliper you cannot compress during a brake job, or a torn dust boot on inspection.
Compressing the piston at a brake job
When you install new, thicker pads, you must push the piston back into its bore to make room. Use a proper tool and go slowly, watching the fluid reservoir. If the piston is seized or the boot is torn, the caliper needs a rebuild or replacement. Fit fresh pads and hardware at the same time. Brakes are safety-critical - if unsure, see a professional.