AN AIRBUS A320 EXPERIENCED A MECHANICAL PROB WITH THE BRAKE AND STEERING CTL UNIT (BSCU) DURING TAXI OUT.

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DURING TAXI OUT WE EXPERIENCED A MINOR MECHANICAL PROB WITH BSCU CHANNEL 2. WE PARKED THE ACFT IN A RUN-UP AREA AND ATTEMPTED TO RESOLVE THE PROB WITH MAINT CTL'S ASSISTANCE. THE BSCU CHANNEL 2 MALFUNCTION COULD NOT BE CLRED; AND MAINT CTL PROPOSED TO DEFER IT. WE WERE AGREEABLE. WITH MAINT CTL ON THE PHONE; WE PROCEEDED WITH THE MEL STEPS NECESSARY TO CONFIRM THAT THE ALTERNATE BRAKING SYS FUNCTIONED PROPERLY; IT WAS DISCOVERED THAT THE ENG #1 ENG-DRIVEN HYD PUMP COULD NOT BE DEPRESSURIZED IF THE ENG WAS ROTATING; AND THE GREEN HYD SYS THUS REMAINED PRESSURIZED (REGARDLESS OF SWITCH AND/OR CIRCUIT BREAKER STATUS). WE RETURNED TO THE GATE FOR MAINT ACTION. 3 MECHS WITNESSED THE SAME PROB; AND THEY COULD NOT DEPRESSURIZE THE ENG #1 ENG-DRIVEN HYD PUMP OR DEPRESSURIZE THE GREEN HYD SYS IF ENG #1 WAS ROTATING. CHANGING THE SWITCH ON THE OVERHEAD PANEL AND CHANGING A CANNON PLUG AT THE PUMP DID NOT CHANGE ANYTHING. A MAINT SUPVR TOLD US THAT HE BELIEVED THAT THIS MIGHT BE DEFERRABLE. I ADVISED HIM THAT TO TAKE THE ACFT IN THIS CONDITION WOULD MAKE IT IMPOSSIBLE FOR US TO COMPLY WITH THE GREEN HYD OVERHEAT IRREGULAR PROC; SHOULD WE HAVE THAT PROB INFLT. SHOULD WE EXPERIENCE A GREEN HYD SYS OR FLUID PROB; EVEN ENG SHUTDOWN WOULD LEAVE THE ENG ROTATING (ABSENT A CORE SEIZURE); AND THUS THE SYS PRESSURIZED THROUGH THE ENG-DRIVEN PUMP. I STATED THAT IF MAINT WERE TO DEFER THIS ITEM AND LEAVE THE ACFT 'AS IS;' I WOULD REFUSE IT. MAINT CONTINUED UNSUCCESSFULLY TO ATTEMPT TO REPAIR THE PROB; AND JUST BEFORE 4 HRS AFTER SCHEDULED DEP THE FLT WAS CANCELED.