MD80 CAPT INADVERTENTLY RELEASED THE PARKING BRAKES DURING AN IRREGULAR OP OF RETURNING TO THE GATE.

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DURING START OF R ENG DURING PUSHBACK; FO STATED L PACK INOP. I TOLD THE GND CREW THAT WE HAD AN ACFT PROB; AND AFTER THE PUSH; TO NOT DISCONNECT. WE STOPPED; AND I SET THE PARKING BRAKE AS REQUESTED; AND TOLD HIM TO 'STAY WITH ME;' WHILE WE SORTED OUT THE PROB. WE CONCLUDED USING THE CHKLIST THAT THE L PACK WAS INOP; A REPEAT ITEM. I CALLED OPS AND WAITED FOR MAINT TO COME ON FREQ. MAINT CONCURRED THAT WE SHOULD RETURN TO THE GATE; AND DURING COORD OF THAT; I TOLD THE TUG DRIVER TO BE PREPARED TO RETURN US TO THE GATE. I FELT KEEPING THE TUG ATTACHED FOR A SUBSEQUENT PUSHBACK AFTER MAINT SIGNED OFF THE BOOK WOULD EXPEDITE OUR DEP. IT WAS AT THIS TIME THAT I INADVERTENTLY RELEASED THE BRAKE; AND THE DRIVER STATED 'STOP -- STOP.' I PRESSED THE BRAKES AND RESET THE PARKING BRAKE. HAVING ADVISED THE DRIVER TWICE PRIOR TO THIS OF A PROB; I FIRMLY BELIEVED THAT THE TUG WAS STILL ATTACHED TO THE ACFT. SUBSEQUENTLY; WE SHOULD NOT HAVE MOVED. THE ACFT DID MOVE SLIGHTLY FORWARD; RESTING SLIGHTLY ON THE R CONNECTOR OF THE TOWBAR. THE TOWBAR HAD BEEN DISCONNECTED. THE GND MAN SIMPLY PULLED THE BAR AWAY FROM THE TIRE WITH INSIGNIFICANT DAMAGE TO THE BAR. NO INJURY TO THE GND CREW; AND NO DAMAGE TO THE TIRE. I THEN CLRED THE GND CREW AND TAXIED TO THE GATE WHERE MAINT FOUND NO TIRE DAMAGE. THEY PLACARDED THE PACK; AND WE CONTINUED TO ZZZ WITHOUT EVENT. I TAKE FULL RESPONSIBILITY FOR THE RELEASING OF THE BRAKE; BUT I WAS CONFIDENT THAT I HAD MADE MY INTENTIONS CLR FOR THEM TO KEEP THE TUG ATTACHED TO THE ACFT PENDING POSSIBLE RETURN TO THE GATE. I WILL INSURE THAT MY INTENTIONS ARE VERY CLR SHOULD A PROB ON PUSHBACK OCCUR AGAIN. NOTE: WHEN ARE THEY ALLOWED TO DISCONNECT THE TOWBAR? THE MANUAL IS CLR THAT THE CAPT WILL ADVISE WHEN TO DISCONNECT INTERPHONE. SUPPLEMENTAL INFO FROM ACN 581160: WE WERE PUSHING BACK FROM THE GATE AND WERE CLRED TO START. AFTER THE L ENG STARTED; I TURNED THE L ACFT SUPPLY SWITCH ON AND DIDN'T SEE A JUMP IN THE LOAD METER. ALSO; THERE WAS NO AIRFLOW ON THE FLOW GAUGE. THE CAPT TOLD THE GND CREW TO STAND BY; BECAUSE WE MIGHT HAVE TO RETURN TO THE GATE FOR MAINT. WE CHKED THE QRH AND DETERMINED THAT THE PACK WAS INOP. I THEN GOT OUT THE MEL TO SEE WHAT WAS REQUIRED TO OPERATE THE FLT. WHILE I WAS READING; THE CAPT INADVERTENTLY RELEASED THE PARKING BRAKE; ASSUMING THAT THE ACFT WAS STILL HOOKED UP TO THE TOWBAR; SINCE HE HAD NOT TOLD THE GND CREW THAT THEY WERE CLRED TO DISCONNECT. UNKNOWN TO HIM; THEY HAD ALREADY DISCONNECTED THE TOWBAR. THE ACFT ROLLED SLIGHTLY FORWARD; RESULTING IN THE NOSE TIRE CONTACTING THE TOWBAR. THE GND CREW YELLED STOP; AND THE CAPT IMMEDIATELY STOPPED THE ACFT. NO ONE WAS INJURED IN THE MISHAP. WE RETURNED TO THE GATE. THE ACFT WAS INSPECTED BY MAINT. THERE WAS NO DAMAGE TO THE ACFT. THE TOWBAR RECEIVED A SLIGHT BEND ON ONE OF THE ATTACHMENT ARMS; BUT WAS STILL SVCABLE.