This O-ring seals the hydraulic pump housing, preventing fluid leakage between the pump body and the mounting face. It sits in the groove at the pump's inlet or outlet port, where compression against the mating surface maintains a closed hydraulic circuit. A degraded or missing O-ring causes external oil seepage and a drop in system pressure, which reduces implement response and can aerate the fluid. The manufacturer supplies this seal to the same cross-section and durometer as the original assembly, so it seats fully in the groove without requiring additional sealant. On reinstall, clean the groove and verify the ring is not twisted before tightening the pump mounting bolts.