What was found
(CAN) FLIGHT CREW CARRIED OUT AN INFLIGHT TURN AROUND DUE TO NR 1 HYDRAULIC ISOLATION CAUTION LIGHT ILLUMINATED AFTER DEPARTURE, NR 1 HYDRAULIC QUANTITY INDICATED 5%. AFTER INTENSIVE TROUBLE SHOOTING PERFORMED BY MAINTENANCE PERSONNEL IT WAS DETERMINED THAT THE NR 1 EDP CAUSE THE PROBLEM (INTERNAL LEAK). EDP REPLACED, FUNCTIONAL CHECK ACCOMPLISHED SATISFACTORY & AIRCRAFT RETURNED TO SERVICE.