As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
GATE RETURN. WHILE TAXING OUT WITH 2 ENGINES RUNNING, PILOT DISCOVERED PARKING BRAKE DID NOT WORK. PILOT REPORTED HYD SYSTEM SEEMED TO BE NORMAL. PILOT CONTACTED MAINTENANCE CONTROL AND WAS ADVISED TO RETURN TO GATE; FOUND PARKING BRAKE CINTROL VALVE AT FAULT. R & R VALVE IAW AMM 32-45-51. PERFORMED LEAK AND OPS CHECK. LEAK AND OPS CHECK GOOD. ACFT OK FOR SERVICE.