What was found
DURING INITIAL TAKEOFF ROLL, AFTER POWER WAS SET TO FLEX, LEFT BRAKES APPEARED TO LOCK UP CAUSING THE ACFT TO LOOSE DIRECTIONAL CONTROL. REJECTED TAKEOFF AND RETURNED TO THE GATE. ACFT PULLED TO LEFT. BRAKES WERE NORMAL ON TAXI BACK TO GATE. THE AIRCRAFT WAS GROUNDED. ACCOMPLISHED 32-51-00-810-831-A, AMM 32-46-00-740-001, 32-42-34-400-001-A, AND 32-42-34-000-001-A. REPLACED BRAKING AND STEERING UNIT. CHECKED NORMAL.