AN ACR WDB ROLLED INTO THE TOWBAR.

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PREFLT NORMAL. L ENG STARTED DURING PUSHBACK. GND CREW REQUESTED 'SET BRAKES' AND CAPT SET THE PARKING BRAKES. HE REPLIED 'BRAKES SET; CLRED TO DISCONNECT.' THE ACFT SLOWLY MOVED FORWARD ABOUT 3 FT WHILE THE PUSHBACK CREW STATED 'SET BRAKES; SET BRAKES!' THE CAPT PUMPED THE BRAKES TO TRY TO STOP THE ACFT. THE HYD PUMPS WERE OFF. INSPECTION OF THE NOSEWHEEL SHOWED A SMALL SIDEWALL CHUNK OF RUBBER GOUGED OUT OF THE L NOSEWHEEL AND THE TOWBAR WEDGED BTWN THE NOSEWHEEL TIRES. THE STATION MGR TOLD US THAT THE PUSHBACK CREW WAS SUPPOSED TO PLACE A CHOCK UNDER THE NOSEWHEEL BEFORE DISCONNECT; WHICH THEY FAILED TO DO. ALSO; A BRAKE ACCUMULATOR SHOULD SUPPLY ADEQUATE BRAKE PRESSURE TO HOLD THE ACFT BEFORE HYD PRESSURE FROM THE HYD PUMPS HAS BEEN TURNED ON. SUGGEST: 1) RECHK HYD BRAKE PRESSURE WHEN SETTING BRAKES (BOTH CREW MEMBERS). 2) REQUEST CONFIRMATION THAT 'NOSEWHEEL IS CHOCKED.' 3) TURN ON HYD PUMPS BEFORE PUSHBACK AND BOTH CREW MEMBERS CONFIRM 'NORMAL HYD BRAKE PRESSURE.' SUPPLEMENTAL INFO FROM ACN 314942: PUMPING THE BRAKES AND SCANNING THE COCKPIT; I DISCOVERED THAT THE HYD SYS SWITCHES WERE NOT ON. I AM NOW EXTREMELY METHODICAL OF ACCOMPLISHING ALL CHKLISTS AND CHK THE BRAKE PRESSURE GAUGE WHEN SETTING THE BRAKES. (SYS DESIGN ALLOWS BRAKE HANDLE TO BE SET WITHOUT ADEQUATE BRAKE PRESSURE TO HOLD THE ACFT.)