This O-ring seals the interface between the hydraulic pump body and its mounting flange. It prevents pressurized hydraulic fluid from leaking past the pump's inlet or outlet ports, maintaining system pressure and preventing aeration. A degraded or pinched O-ring allows external leakage, which reduces pump volumetric efficiency and can draw air into the circuit, causing cavitation damage to the pump's internal components. This component is manufactured to the original cross-section and durometer specification, ensuring it seats fully in the gland without being over-compressed or extruded during installation.