This O-ring seals the hydraulic circuit at the disc-brake assembly. It sits between the stationary housing and the rotating brake disc, preventing fluid leakage from the actuator chamber. A worn or compressed O-ring causes a gradual pressure loss at the brake piston, resulting in reduced clamping force and a spongy pedal feel. This component is manufactured to the original geometry, matching the seal groove depth and cross-section of the factory assembly so that no shimming or adapter is required during installation. Replace the O-ring whenever the brake discs are serviced; lubricate the seal lightly before fitting to prevent rolling in the groove.