This hydraulic oil filter assembly removes particulate contamination from the hydraulic system's return flow before the fluid re-enters the reservoir and the main pump circuit. The set includes the primary hydraulic filter element and a separate breather filter for the reservoir vent. A clogged filter element forces the system's bypass valve open, allowing unfiltered oil to circulate, which accelerates wear on pumps and valves. The manufacturer machines the sealing surfaces to match the housing's bore, ensuring the bypass valve seats correctly and the element seals without leakage. On reinstallation, clean the housing sealing face and replace the O-ring or gasket to maintain a proper seal.