What was found
AIRCRAFT WAS GROUNDED: AIRCRAFT LOST YELLOW HYDRAULIC SYSTEM DURING FLIGHT, AN EMERGENCY WAS DECLARED AND THE AIRCRAFT LANDED SAFELY AT DFW WHERE IT WAS TOWED TO THE GATE WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND THERE WAS NO HYDRAULIC FLUID IN THE SYSTEM, ADDED ONE GALLON IN FOR TROUBLESHOOTING AND FOUND THE #2 ENGINE YELLOW SYSTEM PUMP PRESSURE SWITCH CRACKED AND LEAKING. REPLACED PRESSURE SWITCH , SERVICED THE YELLOW HYDRAULIC SYSTEM AND ACCOMPLISHED A SATISFACTORY LEAK / OPERATIONAL CHECK PER AMM 12-12-29 AND AMM 29-19-17.