This O-ring seals the brake piston inside the brake housing of the host tractor. It prevents hydraulic fluid from bypassing the piston, maintaining the pressure needed for consistent brake application. A worn or hardened O-ring allows internal leakage, which reduces braking force and can cause a spongy pedal feel. As an original AGCO part, the O-ring is molded to the same cross-section and durometer as the original seal, so it seats correctly in the piston groove without requiring shimming or adapters. On reinstall, lubricate the O-ring with clean brake fluid and verify the piston moves freely in its bore before reassembling the housing.