AN AIRBUS A320 AFTER PUSHBACK AND TOWBAR DISCONNECT THE ACFT ROLLED UNCTLED BACKWARDS WITH NORMAL BRAKE SYS INEFFECTIVE.

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WE WERE CLRED TO PUSHBACK IN SJO. PRIOR TO PUSHBACK; WE CONDUCTED AN ALTERNATE BRAKE CHK AS PER MAINT RELEASE DUE TO A DEFERRED BSCU #1 PROB. AT THE COMPLETION OF PUSHBACK; I SET THE PARKING BRAKE AND THE MECH DISCONNECTED THE TOW TRACTOR. THE AIRPLANE THEN BEGAN TO ROLL BACKWARDS. I REPEATEDLY PRESSED THE BRAKE PEDALS AND CYCLED THE PARKING BRAKE HANDLE BUT THE AIRPLANE WOULD NOT STOP. MY FO CYCLED THE ANTI-SKID AND NOSEWHEEL STEERING SWITCH; ALSO WITH NO EFFECT. WE THEN COMMENCED ENG START TO OBTAIN HYD PRESSURE AND FORWARD THRUST. AT THE SAME TIME MY FO XMITTED TO GND CTL THAT WE HAD LOST OUR BRAKES; AND DIRECTED THE FLT ATTENDANTS TO BE IMMEDIATELY SEATED. AS THE #1 ENG SPOOLED UP I AGAIN RELEASED THE PARKING BRAKE AND USED THE BRAKE PEDALS TO STOP THE AIRPLANE SAFELY ON THE TXWY. NO ONE WAS INJURED AND NO DAMAGE OCCURRED. WE THEN HAD THE MECH HOOK THE TOW TRACTOR BACK UP TO OUR ACFT AND TOW US BACK TO THE GATE. AT THE GATE THE MECH EXPLAINED THAT THE BRAKE ACCUMULATOR HAD PROBABLY BEEN PARTIALLY DEPLETED DURING THE OVERNIGHT STAY AND THE ADDITIONAL MAINT RELEASE ALTERNATE BRAKE CHK HAD FURTHER DEPLETED THE ACCUMULATOR TO THE POINT THE BRAKES WOULD NOT HOLD THE AIRPLANE IN PLACE ON THE SLOPED TXWY. AFTER CHARGING THE BRAKE ACCUMULATOR WITH THE YELLOW HYD SYS; WE ACCOMPLISHED ANOTHER ALTERNATE BRAKE CHK; AGAIN RECHARGED THE ACCUMULATOR; RESTARTED THE #1 ENG AT THE GATE; AND COMMENCED AN UNEVENTFUL PUSHBACK. THE A320 FLT HANDBOOK CONTAINS NO EMER PROC OR IRREGULAR PROC FOR BRAKE FAILURE. IT DOES CONTAIN A WARNING TO RELEASE THE PARKING BRAKE IN THE EVENT THE AIRPLANE STARTS MOVING DURING ENG START; BUT DOES NOT ADDRESS THE STARTING OF THE YELLOW HYD SYS ELECTRIC PUMP TO GAIN IMMEDIATE HYD PRESSURE. THE MAINT RELEASE PROC FOR THE ALTERNATE BRAKE CHK DOES NOT CONTAIN A STEP TO RECHARGE THE BRAKE ACCUMULATOR AFTER PERFORMING THE CHK. 1) ADD A320 LOSS OF BRAKES EMER PROC TO INCLUDE: A) PARKING BRAKE -- RELEASE. B) YELLOW HYD SYS ELECTRIC PUMP -- ON. 2) ADD NEW STEP TO END OF A320 MAINT RELEASE ALTERNATE BRAKE CHK: RECHARGE BRAKE ACCUMULATOR WITH YELLOW HYD SYS ELECTRIC PUMP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE REQUIRED CHK FOR THE DEFERRED ITEM DEPLETED THE YELLOW SYS ACCUMULATOR AND WAS NOT REPLENISHED WITH THE YELLOW ELECTRIC HYD PUMP. THE APU WAS RUNNING AND #1 ENG WAS STARTED AND SUPPLIED HYD PRESSURE FOR THE NORMAL BRAKING SYS.