A B767-300 GND CREW CLRED FOR PUSHBACK AFTER TFC CLRED. CAPT INADVERTENTLY RESET PARKING BRAKE WHILE WAITING TO COMPLETE PUSHBACK CAUSING TOWBAR SHEAR PINS TO SHEAR.

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I RELAYED THE CLRNC TO OUR GND AND TUG CREW THAT WE WERE CLRED TO PUSH AFTER THE TFC HAD CLRED BEHIND US. IT TOOK A WHILE TO MOVE AND I MAY HAVE RESET THE BRAKE. (I ALWAYS SET THE BRAKES DURING TAXI WHEN WE STOP; BUT NEVER DURING PUSHBACK.) GND COPIED AND COMPLIED WITH THE CLRNC. I WAS REALLY SURPRISED THAT THE BRAKES WERE NOT RELEASED. I COULD NOT RECALL IF I DID RELEASE OR IF I RESET THEM AFTER THEY WERE RELEASED. I READ THE RECENT RPTS ABOUT BRAKE RELEASE RELATED INCIDENTS AND TOWBARS; AND WONDERED HOW; AND WHY WE WERE MAKING THESE MISTAKES. LUCKILY NO ONE WAS INJURED; AND THE DAMAGE WAS LIMITED TO THE SHEAR PINS ON THE TOWBAR. I THINK WHAT CONTRIBUTED TO THIS ERROR ARE A COMBINATION OF FACTORS. OUR OBSESSION WITH ON TIME DEP; OUR RELIEF PLT ADVISED ME THAT IT WAS SCHEDULE DEP; AND ASKED IF I WANTED TO GIVE THE ON TIME OUT; AS OUR FO WAS GIVING ME THE PUSHBACK CLRNC. (HE LATER MENTIONED THAT IT MAY HAVE DISTR MY NORMAL FLOW.) GND CREW STARTS TO PUSH; AFTER A DELAY WAITING FOR THE TFC TO CLR; WITHOUT STATING; 'WE ARE STARTING THE PUSH'; (EVEN THOUGH I MAY HAVE CONFIRMED THAT PARKING BRAKES WERE RELEASED) PRIOR TO PUSH. SHOULD AMEND PROC SO THAT GND CREW REQUIRED TO VERBALLY RESTATE 'STARTING THE PUSH' ANY TIME THERE IS ANY DELAY DURING THE OP. THE GND CREW WAS SUPERB; HIGHLY PROFESSIONAL; THEY GOT US OUT WITH A MINIMUM DELAY; AND WITHIN MINS WE COMPLETED THE PUSHBACK. WE GOT BACK TO DEST ON SCHEDULE.