What was found
RETURNED TO DEPARTURE, DUE TO A HYDRAULIC SYSTEM A ENGINE PUMP LOSS. PUMP WOULD NOT DEPRESSURIZE IN FLIGHT. AN EMERGENCY WAS DECLARED AND AIRCRAFT LANDED WITHOUT INCIDENT. AIRCRAFT WAS GROUNDED. ENGINE HYDRAULIC PUMP DOES NOT DEPRESSURIZE, ENGINE 1 HYDRAULIC PUMP READS LOW PRESSURE, FOLLOWED BY SYSTEM A QUANTITY LOSS TO 14%,. WHILE SERVICING SYSTEM A HYDRAULIC DRIVEN PUMP FOUND TO BE LEAKING FROM DRAIN MAST. REPLACED LEFT ENGINE PUMP, RETURN, SUPPLY AND CASE DRAIN LINES IAW AMM 29-11-11 AND SYSTEM A ASSOCIATED FILTERS. ALSO ACCOMPLISHED SYSTEM A FLUSH AND FILL IAW MCM CARDS 29-93-01 AND 29-93-02. ALL OPERATIONS AND LEAK CHECKS ARE GOOD.