ACR HITS JETWAY WHEN PARKING BRAKE IS SET. NO PRESSURE WAS INDICATED.

Narrative

UPON ARR AT MEX GATE; THE ACFT WAS BROUGHT TO A COMPLETE STOP AND THE PARKING BRAKE WAS SET. THE FO AND I THEN PROCEEDED WITH THE POSTFLT DUTIES. SHORTLY THEREAFTER; THERE WAS A BANGING ON THE SIDE OF THE FUSELAGE. THE FO AND I BOTH LOOKED UP AND NOTICED THE GUIDEMAN SIGNALING FOR A STOP. THE FO AND I BOTH APPLIED MAX BRAKING AND BROUGHT THE ACFT TO A STOP. WHILE HOLDING THE BRAKES; I VERIFIED THE PARKING BRAKE WAS ON AND THEN LOOKED AT THE BRAKE PRESSURE INDICATOR; WHICH SHOWED NORMAL ACCUMULATOR PRESSURE BUT ZERO PRESSURE FOR THE PARKING BRAKE. I THEN RELEASED THE PARKING BRAKE AND THEN RESET IT AND VERIFIED NORMAL PARKING BRAKE PRESSURE. FURTHER INVESTIGATION REVEALED THAT THE ACFT ROLLED FORWARD APPROX 15-20 FT BEFORE IT WAS STOPPED. DURING THE MOVEMENT THE L ENG COWLING CONTACTED THE JETWAY AND RECEIVED AN APPROX 3 INCH GASH ON THE TOP OF THE COWL. CONTRIBUTING TO THIS INCIDENT WAS THE FACT THAT I WAS MOMENTARILY DISTR WHEN SETTING THE PARKING BRAKE. MY ATTN WAS DIVERTED AWAY WHEN THE FO QUERIED ABOUT SHUTTING DOWN THE #2 ENG. I LOOKED AWAY FROM THE BRAKE GAUGE TO VERIFY THAT THE APU WAS AVAILABLE. THEREFORE; I WAS NOT ABLE TO VERIFY PROPER PARKING BRAKE PRESSURE WHEN IT WAS INITIALLY SET. CLRLY FULL ATTN SHOULD BE DEVOTED TO THE MOST IMPORTANT TASK AT HAND; PRIOR TO MOVING ON TO THE NEXT ONE.