What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED ON FINAL APPROACH INTO DFW DUE TO THE FLIGHT CREW REPORTING THAT AFTER LOWERING THE GEAR A YELLOW HYDRAULIC SYSTEM RESERVOIR LOW LEVEL ECAM MESSAGE DISPLAYED AND THAT ALL HYDRAULIC FLUID WAS LOST FOR THE YELLOW SYSTEM. THE AIRCRAFT LANDED SAFELY AT DFW AND TAXIED TO THE GATE UNDER ITS OWN POWER. MAINTENANCE FOUND THE ALTERNATE BRAKE DUAL DISTRIBUTION VALVE HAD A SHEARED UNION AND REPLACED IT PER AMM 32-43-14. A SATISFACTORY LEAK / OPERATIONAL CHECK WAS ACCOMPLISHED.