What was found
(CAN) ON 18-JUNE-2019, DURING CRUISE, THE #1 HYD ISO VLV CAUTION LIGHT ILLUMINATED. THE HYD QUANTITY IN #1 HYD SYSTEM WAS OBSERVED AT 15% WHICH SUBSEQUENTLY DROPPED TO 0 % FOLLOWED BY THE #1 ENG HYD PUMP CAUTION LIGHT AND THE #1 HYD VALVE CLOSED ADVISORY LIGHT. THE FLIGHT CREW DECLARED A MAYDAY CALL AND LANDED AT THE ALTERNATE AIRPORT DUE TO A GREATER RUNWAY LENGTH AVAILABLE. THE MAINTENANCE CREW INSPECTED THE AIRCRAFT AND REPLACED THE LH EDP IAW AMM 29-11-01. THE LH FILTER MANIFOLD PRESSURE, CASE DRAIN FILTERS WERE ALSO REPLACED IAW AMM 29-11-31. THE HYDRAULIC LOCK ON THE PX FILTER BOWL WAS RESOLVED IAW CMM 29-12-04. THE AIRCRAFT RETURNED TO SERVICE WITHOUT ANY FURTHER ISSUES. (TC# 20190621013)