What was found
(CAN) LOSS OF MAIN HYDRAULIC FLUID DURING FLIGHT DUE TO GROUND HYDRAULIC TEST CONNECTION (P/N 230566-8) COMING LOOSE AND SEPARATING FROM ADAPTER FITTING (P/N 22336699) THAT IS ATTACHED TO PIPE ASSEMBLY (P/N 5321900-26) HYDRAULIC PRESSURE LINE IN #3 ENGINE NACELLE. SUSPECT CAUSE ATTRIBUTED TO DETERIORATION OF THE "O" RING RESULTING IN THE LOSS OF TORQUE FRICTION ALLOWING THE CHECK VALVE TO WORK LOOSE UNDER VIBRATING CONDITIONS.