A BEECH STARSHIP PROX ALERTS THE TCASII OF A B757; CREATING AN EVASIVE ACTION MANEUVER ON THE PART OF THE B757 CREW WHILE ON A VISUAL APCH TO SFO; CA.

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WE WERE ON VECTORS TO EXPECT A VISUAL APCH TO RWY 28R AT SFO ARPT. ON A HDG OF 180 DEGS WE WERE ADVISED OF TFC; A B737 AND A B757. WE HAD BOTH ACFT IN SIGHT AND RPTED THIS. WE WERE THEN ISSUED A VISUAL APCH TO RWY 28R. AT THE TIME OF THE ISSUANCE OF THE VISUAL APCH WE WERE JUST RECEIVING THE LOC NEEDLE AND PROCEEDED TO TURN R TO RWY 28R. WE HAD A B757 IN SIGHT THROUGH THE ENTIRE APCH AND MAINTAINED VISUAL SEPARATION. THE B757 WAS ABOVE AND TO OUR L AND DSNDED ABEAM US AS WE SLOWED TO OUR LNDG GEAR EXTENSION SPD. THE B757 WAS SLIGHTLY AHEAD AND TO OUR L. AT THIS TIME; THEY (B757) TEMPORARILY BROKE OFF THE APCH DUE TO A TCASII WARNING. AT NO TIME WERE ANY ACFT IN DANGER. I FEEL THAT THE B757 USED APPROPRIATE ACTION IN BREAKING OFF THEIR APCH. IT IS UNCLR TO ME IF THEY HAD A VISUAL ON US. AT NO TIME DID WE LOSE SIGHT OF THE B757. I FEEL THAT IF WE WERE ISSUED A VECTOR EARLIER TO THE FINAL APCH COURSE AND/OR LESS THAN THE 100 DEG TURN THAT WE RECEIVED TO INTERCEPT THE FINAL APCH COURSE; THIS PROB MAY NOT HAVE OCCURRED.