Inside every brake caliper, a small square-cut rubber ring called the piston seal does two clever jobs at once. When it fails, you get leaks or a dragging brake. Here is what the caliper piston seal does and why it matters.
Two jobs in one seal
The piston seal sits in a groove in the caliper bore, around the piston. Its obvious job is sealing high-pressure brake fluid so none leaks past the piston. Its clever second job: the square-cut seal flexes slightly when you brake, then springs back when you release, pulling the piston back a hair - this is what gives the pad a tiny clearance so it does not drag. It is called seal roll-back.
How it fails
Over years, the rubber seal hardens, cracks, or swells - especially with old, moisture-laden fluid that degrades the rubber, another reason to change fluid on schedule. A failed seal can leak fluid past the piston (a dangerous brake fluid leak) or lose its roll-back so the pad drags and the brake sticks - see sticking caliper symptoms.
The dust boot works with it
A separate dust boot keeps water and grit off the piston and out of the seal area. When the boot tears, corrosion reaches the piston and seal, accelerating failure and a caliper that won't retract. A torn boot is an early warning to act.
Repair options
A failed seal can be replaced with a caliper rebuild (seal kit) if the bore and piston are clean, or the caliper replaced - see rebuild vs replace. Keep fluid fresh to protect the seals, and fit quality pads and hardware when servicing.