This pump cover O-ring seals the interface between the pump housing and its cover, preventing hydraulic fluid leakage at the joint. It sits in the groove machined into the pump cover face, where it is compressed when the cover is bolted down. A degraded or hardened O-ring allows external seepage or air ingress, which can cause cavitation and erratic pump output. As an original AGCO part, the O-ring is molded to the same cross-section and durometer as the seal originally fitted to the assembly, so it seats fully in the groove without requiring additional sealant. On reinstallation, clean the groove surface and verify the O-ring is not twisted before torquing the cover bolts to specification.