What was found
FLT 0035 - AFTER TAXI TO RUNWAY & SETTING THE PARKING BRAKES, BRAKE PRESSURE DROPPED TO 1150, RETURNED TO GATE. MAINTENANCE REMOVED AND REPLACED THE PARKING BRAKE ACCUMULATOR IAW BMM 32-44-05, AND HYDRAULIC ALTERNATE BRAKE SELECT VALVE IAW 32-41-04. OPERATIONAL AND LEAK CHECKS WERE GOOD. TEARDOWN OF HYD BRAKE SELECT VALVE, CONFIRMED FAILURE OF UNIT, PROBLEM WAS IDENTIFIED AS WORN O-RINGS AND SEALS. UNIT WAS OVERHAULED, TESTED, AND RETURNED TO SERVICE.