What was found
AIRCRAFT WAS GROUNDED: DIVERTED TO JFK FOR L HYD INSIDIOUS LEAK THAT WAS DROPPING .01 OVER 10 MIN. CALLED MOC ENROUTE AND WITH HISTORY OF HYD ISSUE DEEMED UNSAFE TO CONTINUE TO LHR DIVERTED TO JFK. EMERGENCY WAS DECLARED AND AIRCRAFT LANDED WITHOUT INCIDENT AND TAXIED TO GATE. MAINTENANCE COULD NOT FIND ANY VISABLE LEAKS. SERVICED L/H HYD SYSTEM AS REQUIRED ACCOMPLISHED MAIN HYDRAULIC SYSTEMS EXTERNAL LEAKAGE CHECK REF AMM 29-11-00-200-804 NO LEAKS NOTED AT THIS TIME, OK FOR SERVICE.