What was found
(CAN) HYDRAULIC SYSTEM NR 1 FAILED IN FLT (AFTER TAKE OFF), LOW QUANTITY. THE FLIGHT CREW DECLARED AN EMERGENCY AND RETURNED TO PLACE OF DEPARTURE. MAINTENANCE FOUND THAT THE HYDRAULIC SYSTEM FAILURE WAS CAUSED BY A PACKING FAILURE OF THE THERMAL RELIEF VALVE ON THE PRESSURE MANIFOLD ASSY, THE VALVE WAS REPLACED (PN OFF ON: RV99-199, SN OFF 1011927, SN ON 04-0916528) IAW AMM 32-44-13. THE SYSTEM WAS REPLENISHED AND TESTED SERVICEABLE.