What was found
(CAN) PROP REPLACED ON AIRCRAFT AFTER TROUBLESHOOTING PROPELLER CONTROL SYSTEM AND INSPECTING OIL PUMP AND LINES. NO DEFECTS FOUND IN PUMP, LINES OR WITH GOVERNOR ASSY. REPLACING PROP SOLVED PROBLEM. DISASSEMBLY REVEALED DAMAGED SEAL IN FRONT OF PITCH CHANGE PISTON CAUSING OIL PRESSURE TO BUILD UP ON BACKSIDE OF PISTON, AND FORCING PROP TO COARSE PITCH. THIS OIL PRESSURE WAS GREATER THAN RETURN SPRING PRESSURE AND THE HUB HAS NO DUMP PROVISION IN THIS AREA TO ALLOW OIL TO RETURN TO THE ENGINE.