What was found
NORMAL AND PARKING BRAKES FAILED AT GATE. NORMAL BRAKES WERE APPLIED TO STOP AIRCRAFT, PARKING BRAKE WAS SET & STARTED TO SHUTDOWN NR 2 ENG WHEN AIRCRAFT STARTED TO ROLL. NORMAL BRAKES REAPPLIED & PARKING BRAKE RESET, AIRCRAFT STOPPED THEN STARTED TO ROLL AGAIN. NORMAL BRAKES APPLIED WITH NO EFFECT, RECYCLED PARKING BRAKE WHILE FIRST OFFICER SHUTDOWN NR 1 ENG. AIRCRAFT WAS CHOCKED AND STOPPED ROLLING. MAINTENANCE TROUBLESHOT THE BRAKE SYSTEM AND FOUND GREEN HYDRAULIC SYSTEM MIGRATING TO YELLOW HYDRAULIC SYSTEM. DURING T/S FOUND THERE WAS NO BRAKE PRESSURE WITH THE NR 1 ENG AND ONLY THE GREEN HYD SYS BEING USED. REMOVED AND REPLACED AUTOMATIC SELECTOR VALVE, OPERATIONAL AND LEAK CHECKS WERE GOOD.