ACFT HAS DIFFICULT TASK STAYING BEHIND A B-757 ON A VISUAL APCH AND EVENTUALLY PASSES HIM ON PARALLEL RWY APCH INTO SFO.

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ON A RECENT FLT INTO SFO I WAS CLRED FOR THE QUIET BRIDGE 28L VISUAL APCH. AFTER LNDG I WAS ASKED TO TELEPHONE BAY APCH CTL. UPON SO DOING THE SUPVR I SPOKE WITH INFORMED ME THAT MY RADIO XMISSIONS FOLLOWING THE APCH CLRNC HAD BLOCKED THE CTLR'S XMISSIONS 3 TIMES; MAKING HIS JOB DURING A VERY BUSY PERIOD MUCH HARDER. I AM SURE THAT THIS WAS TRUE; THOUGH CERTAINLY NOT MY INTENT. WHAT WERE THE CIRCUMSTANCES LEADING UP TO THIS SIT? WHILE BEING VECTORED FOR THE APCH BAY APCH CTL ASSIGNED A SPD OF 180 KTS. THEY THEN ASKED IF I HAD A 757 IN SIGHT AT 2 O'CLOCK. I REPLIED THAT I HAD AN AIRPLANE VISIBLE AT 1:30 HIGH. THIS WAS APPARENTLY GOOD ENOUGH FOR THE CTLR; FOR HE THEN CLRED ME FOR THE TIPP TOE VISUAL 28L APCH; 'MAINTAIN VISUAL SEPARATION FROM THE 757 LNDG RWY 28R.' KNOWING THAT; ONCE CLRED FOR A VISUAL APCH I WOULD BE RESPONSIBLE FOR MAINTAINING WAKE TURB SEPARATION FROM THIS TFC; I BECAME CONCERNED THAT THE 757 MIGHT END UP AHEAD OF MY PLANE. SINCE I HAD BEEN ASSIGNED A SPECIFIC SPD (TO BRIJJ; I BELIEVE) I FELT RATHER BOXED IN AS TO OPTIONS TO MAINTAIN SAFE SEPARATION FROM THE 757. EACH TIME I ATTEMPTED TO ASK THE CTLR ABOUT THE 757'S SPD OR INTENTIONS I BLOCKED THE FREQ. AS THE APCH PROGRESSED IT BECAME APPARENT THAT THE 757 WAS IN FACT GOING BEHIND US. SINCE THE FO WAS FLYING THE APCH; IT BECAME EVER MORE DISTRACTING FOR HIM TO LOOK BACK OVER HIS SHOULDER WHILE BEING CAREFUL NOT TO NEGLECT HIS NAV AND RISK OVERSHOOTING THE TURN ONTO FINAL FOR RWY 28L. I COULD NOT SEE THE 757 AT ALL. ALTHOUGH THE TCASII DISPLAY WAS HELPFUL; THIS WAS NEITHER ITS INTENDED FUNCTION NOR A SUBSTITUTE FOR VISUAL CONTACT. I SUPPOSE THAT IF I HAD TRIED TO TELL THE CTLR THAT I COULD NO LONGER MAINTAIN VISUAL SEPARATION I MIGHT HAVE BLOCKED HIS BUSY FREQ A 4TH TIME. DURING MY PHONE CONVERSATION THE SUPVR REPEATEDLY EMPHASIZED THAT I SHOULD NOT QUESTION THAT THE CTLR WOULD DO HIS JOB CORRECTLY; THAT IS; TO SET THE INITIAL CONDITION OF WAKE VORTEX SEPARATION. BUT WHAT ABOUT SEPARATION AFTER THE VISUAL CLRNC IS ISSUED? THE MATTER OF MAINTAINING SAFE WAKE TURB SEPARATION IS BUT ONE OF MANY PROBS BROUGHT ABOUT BY THE MISUSE OF VISUAL APCH PROCS AT SFO. AS A FORMER CHAIRMAN OF THE LCL ATC COMMITTEE I AM ALL TOO AWARE OF THE LONG-STANDING PROBS WITH THE USE OF SIMULTANEOUS VISUAL APCHS TO RWYS 28 AT SFO. NEVERTHELESS; I MUST ASK ONCE AGAIN THAT THE FAA TREAT THIS AS A SINGLE RWY FOR THE PURPOSE OF VISUAL APCH CLRNCS. IN THIS MANNER EACH PLT WILL BE TOLD SIMPLY TO FOLLOW THE PRECEDING AIRPLANE TO THE ARPT. THUS IT WILL BE CLR THAT; PROVIDED THE PLT HAS RPTED THE PRECEDING AIRPLANE IN SIGHT: 1) HE MUST FOLLOW (IE; NOT PASS) THAT AIRPLANE. 2) HE MUST AVOID THE PRECEDING AIRPLANE; EVEN IF THAT PLT OVERSHOOTS THE TURN TO FINAL OR BANKS TO PREVENT A COLLISION WITH BIRDS; BALLOONS; OR ANYTHING ELSE. 3) HE MUST MAINTAIN SAFE WAKE TURB SEPARATION; SINCE THE CTLR WON'T.