As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON TRYING TO TAXI, RELEASED BRAKES A/C DIDN'T MOVE THEN A LOUD CLUNK AND THEN A/C MOVED AND ALL FLIGHT DIRECTOR ITEMS WENT AWAY ON BOTH PFD'S WITH THE MESSAGE, FD MODE CHANGE. SET PARK BRAKES AND RE-RELEASED THEM AND A/C WOULD NOT MOVE. CYCLED THE PARK BRAKE AGAIN AND THE PARK BRAKE RELEASED. TAXIED BACK TO THE GATE. REMOVED & REPLACED PARKING BRAKE CONTROL VALVE IAW AMM 32-44-01-000-801/400-801. TAXIED A/C AROUND, NO ISSUE BRAKING OR LOUD NOISES. RETURNED TO SERVICE.