What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO PHL DUE TO THE FLIGHT CREW REPORTING THAT THE YELLOW HYDRAULIC SYSTEM QUANITY WAS LOW APPROXIMATLEY 10 MINUTES AFTER TAKEOFF. THE FLIGHT CREW REPORTED THAT THE QUANTITY STABILIZED WITH ENGINE DRIVEN PUMP OFF AND WHEN IT WAS TURNED BACK ON PER THE QRH FOR THE APPROACH YELLOW SYSTEM BLEED OUT AND PUMP WAS TURNED OFF. THE AIRCRAFT LANDED SAFELY AT PHL WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND THE RIGHT ENGINE YELLOW HYDRAULIC ENGINE DRIVEN PUMP PRESSURE LINE WAS CHAFED THROUGH AND LEAKING. MAINTENANCE REPLACED HYDRAULIC LINE AND ACCOMPLISHED A SATISFACTORY LEAK / OPERATIONAL CHECK PER AMM 29-13-49.