A320 DECIDES TO DEPART GATE WITH HOT BRAKES IN ANTICIPATION OF THEIR COOLING PRIOR TO TKOF. RWY INCURSION RESULTS WHILE HOLDING BRAKES MANUALLY AT HOLD SHORT LINE.

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WHILE DOING PREFLT CHKS; WITH ABOUT 25 MINS PRIOR TO DEP; I NOTICED THE #1 AND #2 BRAKE TEMPS WERE OVER 300 DEGS (THE #2 BRAKE WAS OVER 400 DEGS). THE FO AND I COORD CHOCKS (WHICH TOOK SOME TIME) AND THE #2 BRAKE WAS JUST BELOW 300 DEGS AT PUSHBACK TIME. I DECIDED TO PUSH BACK AND TAXI; USING ONLY THE R BRAKE. DURING THE COURSE OF TAXIING; WHICH INVOLVED A RWY CHANGE DUE TO A MICROBURST (FROM RWY 25 TO RWY 34L) AND A PROB WITH LOAD PLANNING; THE #3 AND #4 BRAKES WENT OVER 300 DEGS. TWR ASKED US TO HOLD SHORT OF RWY 34L AT TXWY D2 TO LET OTHERS BEHIND US TAKE OFF. I ELECTED TO NOT SET THE PARKING BRAKE WHILE HOLDING SHORT; BUT ONLY HOLD THE L BRAKE SO THE R SIDE COULD COOL FASTER. THAT WAS MY MISTAKE. WHILE TALKING WITH THE FO ABOUT OUR LOAD PLANNING ISSUE; COUPLED WITH IT BEING DARK OUTSIDE AND THE DOWNHILL SLOPE OF THE TXWY TOWARD THE RWY; THE ACFT CREPT FORWARD WITHOUT MY NOTICING IT. WHEN THE FO NOTICED; WE WERE ABOUT 4 FT OVER THE HOLD SHORT LINE. SHORTLY AFTER WE NOTICED THE INCURSION (WE HAD PREVIOUSLY TOLD TWR WE WERE READY TO GO BECAUSE OUR BRAKES HAD COOLED); WE WERE CLRED FOR TKOF.