This O-ring seals the piston bore in the rear disc brake caliper. It prevents hydraulic fluid from leaking past the piston when the brake pedal is applied and released. A worn or hardened O-ring causes a slow fluid weep that reduces clamping force and can lead to a spongy pedal feel. The ring is sized to the original caliper groove geometry so the piston retracts fully when pressure is released. On reinstall, lubricate the ring with clean brake fluid and verify the piston moves freely before bleeding the circuit.