What was found
DURING TAXI OUT WITH FLAP TRANSITION, STEERING WOULD NOT OPERATE DUPLICATED STEERING PROBLEM BY OPERATING SLATS & FLAPS & STEERING AT THE SAME TIME. HYD PRESS DROPPED BELOW THRESHOLD OF THE PRIORITY VALVE ON THE FLAP SLAT DEPRESS MODULE, TURNING STEERING SYS M OFF. REPLACED ENG DRIVEN HYD PUMP IAW MM CH 29-11-05-4-1& PRESS & CASE DRAIN FILTERS IAW MM CH 29-11-17-4. RAN ENG IAW JRH & HYD DO NOT HAVE SIGNIFICANT PRESS DROP, LEAK CHECKS OK.