What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO DFW DUE TO THE FLIGHT CREW REPORTING AT CRUISE A BLUE HYDRAULIC SYSTEM RESERVOIR LOW LEVEL ECAM MESSAGE AND THE LOSS OF ALL BLUE HYDRAULIC SYSTEM FLUID. THE AIRCRAFT LANDED SAFELY AT DFW AND TAXIED TO THE GATE UNDER ITS OWN POWER. MAINTENANCE FOUND THE BLUE HYDRAULIC SYSTEM PRESSURE SWITCH 2074GJ LEAKING DUE TO A BROKEN O-RING. MAINTENANCE REPLACED THE O-RING AND ACCOMPLISHED A SATISFACTORY LEAK/OPERATIONAL CHECK.