B737 ON APCH TO SFO RWY 28L EXECUTES GAR WHEN OVERTAKEN BY B757 ON APCH TO RWY 28R.

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WE WERE CLRED FOR A VISUAL APCH TO RWY 28L AT SFO. BAY TRACON POINTED OUT A B757 WHICH WAS APCHING RWY 28R. WE WERE PAIRED UP WITH THE B757 AND WERE TOLD THAT HE WAS MAINTAINING VISUAL SEPARATION WITH US. WE WERE WELL AHEAD OF THE B757 AND ACCORDING TO BAY TRACON; OUR SPDS WERE MATCHED. BAY APCH HAD ASSIGNED US 180 KIAS TO THE SAN MATEO BRIDGE; WHICH WE MAINTAINED. AFTER PASSING THE BRIDGE WE STARTED SLOWING TO OUR FINAL APCH SPD. WE NOTICED THE B757 GAINING ON US; STILL CONFIGURED GEAR UP. THE B757 DID NOT LOWER HIS LNDG GEAR UNTIL BELOW 900 FT AGL AND WAS STILL GAINING ON US. OUR FINAL APCH SPD WAS 140 KIAS AND THE FO (PF) WAS CONFIGURED AND STABILIZED BY 1000 FT AGL. AT APPROX 500 FT AGL; THE B757 STARTED TO OVERTAKE US AT A RAPID RATE AND WE HAD NO CHOICE BUT TO GO AROUND FOR WAKE TURB AVOIDANCE. THIS WAS OUR SECOND GAR FOR THIS FLT. THE FIRST ONE WAS CAUSED BY INSUFFICIENT SEPARATION WITH A RWY 1R DEP AND WAS INITIATED BY SFO TWR. THE EXTRA FUEL ON THIS SEGMENT SIGNIFICANTLY REDUCED THE STRESS LEVEL IN THE FLT DECK. THIS IS THE SECOND TIME IN ABOUT 4 YRS THAT I'VE PERFORMED BACK-TO-BACK GAR'S AT SFO DUE TO LACK OF SEPARATION WITH HVY ACFT WHILE MAINTAINING ATC ASSIGNED SPDS.