What was found
PILOT SELECTED GEAR DOWN ON APPROACH AND NOTHING HAPPENED. ABORTED APPROACH AND GAINED SAFE ALTITUDE. THEN RAN THE EMERGENCY GEAR EXTENSION CHECKLIST USING THE EMERGENCY HAND PUMP AND RECEIVED THREE GREEN GEAR DOWN AND LOCKED LIGHTS AND PROCEEDED WITH A NORMAL LANDING. UPON MECHANICS INSPECTION, FOUND LEFT HYDRAULIC SYSTEM FILTER LEAKING AND LOSS HYDRAULIC OIL IN RESERVOIR. AFTER REMOVING FILTER, FOUND FILTER HOUSING FRACTURED AT THREADS. APPROXIMATE TIME SINCE LAST INSPECTION WAS APPROXIMATELY 150 HOURS. MECHANIC INSTALLED A USED SERVICEABLE FILTER HOUSING AND NEW FILTER ELEMENT AND PRESSURE TESTED SYSTEM. LANDING GEAR FUNCTIONED NORMALLY.