AN AIRBUS A-320 ACFT BRAKE STEERING CONTROL UNIT (BSCU) WOULD NOT RELEASE FROM THE PARKING 'BRAKE SET' POSITION WHILE HOLDING SHORT FOR TAKE-OFF.

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NORMAL PREFLT AND TAXI-OUT UNTIL REACHING HOLD SHORT POINT FOR RWY 27R AT TXWY E. SET PARKING BRAKE WAITING ON CLRNC TO CROSS 27R. WHEN CLRED TO CROSS PARKING BRAKE WOULD NOT RELEASE. ALL INDICATIONS IN COCKPIT WERE THAT THE PARKING BRAKE WAS RELEASED; IE; PARK BRAKE MESSAGE ON ECAM DISAPPEARED AND TRIPLE GAUGE NEEDLES WENT TO ZERO. TRIED RESETTING BSCU AND VARIOUS CIRCUIT BREAKERS WITH MAINT CTLR TO NO AVAIL. CONTRACT MAINT CALLED OUT AND HE TRIED TO GET BRAKES OFF WITH NO SUCCESS. DEPLANED PAX WITH PEOPLE MOVER APPROX 90 MINS AFTER TAXI OUT. MECHS CAME ON DUTY AND OUT TO ACFT AND THEY DISCONNECTED BRAKE LINES AND THE ACFT WAS TOWED TO GATE. IT TOOK ABOUT 2 HRS 50 MINS FROM TAXI OUT TILL BACK AT GATE. KEPT ATC IN LOOP AT ALL TIMES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PILOT DOES NOT KNOW THE FIX FOR THE 'LOCKED' BRAKES CONDITION. HE HAS HEARD OF REPORTS FROM OTHER PILOTS ABOUT BRAKES LOCKING-UP BUT ATTRIBUTED THESE TO PREVIOUS ISSUES HE THOUGHT WERE RESOLVED. HE WAS CONCERNED ABOUT THE LEVEL OF EXPERIENCE OF THE CONTRACT MECHANIC WHO INITIALLY CAME OUT TO THE ACFT AND WAS HAMMERING ON THE BRAKE SHUTTLE VALVE PRIOR TO COMPANY MECHANICS ARRIVING ON THE SCENE. THE ACFT WAS ALSO COMPLETELY POWERED DOWN AND RE-POWERED UP; HOPING THIS MIGHT RESET THE SYSTEM AND UNLOCK THE BRAKES; BUT THIS DID NOT HELP. REPORTER REITERATED AGAIN THAT THE PARK BRAKE MSG ON ECAM HAD DISAPPEARED AND THE TRIPLE GAUGE NEEDLES WENT TO ZERO INDICATING THE PARKING BRAKE WAS RELEASED; BUT WAS; IN FACT; STILL PHYSICALLY SET. REPORTER ALSO NOTED THE ACFT HAD SETTLED APPROX 3-4' INCHES INTO THE TARMAC BEFORE THE ACFT WAS FINALLY MOVED OFF THE TAXIWAY.