FLC OF A B737-300 FAILED TO ADVISE ATC WHEN LOSING VISUAL SIGHTING OF A PRECEDING B737 AHEAD ON A VISUAL APCH TO A PARALLEL RWY. THERE WAS NO KNOWN CONFLICT.

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DURING APCH TO SFO; JUST AFTER PASSING MENLO INTXN; WE WERE CLRED FOR A VISUAL APCH TO SFO RWY 28L. THIS CLRNC WAS RECEIVED AFTER CONFIRMING 'ARPT IN SIGHT.' BAY APCH POINTED OUT COMPANY ACFT APPROX 1000 FT HIGHER AND 8-10 MI TURNING R BASE FROM DOWNWIND. WE ACKNOWLEDGED 'IN SIGHT.' I DON'T BELIEVE WE WERE TOLD TO 'MAINTAIN VISUAL SEPARATION' BUT I'M NOT SURE. IN ANY CASE ATC TOLD US TO SLOW FROM 250 KTS TO 210 KTS BECAUSE WE WERE '100 KTS FASTER' THAN TFC WE WERE FOLLOWING. THEY SUBSEQUENTLY SLOWED US TO '180 KTS TO THE BRIDGE.' AS A RESULT OF THE ASSIGNED SPD; WE WERE UNABLE TO STAY BEHIND THE PARALLEL TFC; AND I ASKED THE COPLT IF HE COULD SEE THE TFC AS WE CROSSED THE HIGH SPAN 'BRIJJ.' HE INDICATED HE DID NOT HAVE HIM IN SIGHT. WE SHOULD HAVE ADVISED SFO TWR AT THAT TIME AND EXECUTED A MISSED APCH. HOWEVER; WE HAD HIM ON TCASII AND DISTANCE WAS NOT A FACTOR; SO I ELECTED TO CONTINUE. WHILE THIS WAS NOT AN INCIDENT PER SE (IT HAPPENS AT SFO ON A REGULAR BASIS; MANY TIMES A DAY). CONTRIBUTING FACTORS TO THIS DEV FROM FARS AND COMPANY SOP WERE AS FOLLOWS: 1) AMBIGUOUS CLRNC FROM ATC; IE; VISUAL APCH AND MAINTAIN 180 KTS -- WERE WE ASSIGNED TO 'MAINTAIN VISUAL SEPARATION' OR WAS THE OTHER ACFT? 2) WE SHOULD HAVE CLARIFIED THE CLRNC WITH ATC AND DETERMINED WHO WAS TO MAINTAIN SEPARATION. 3) THIS WAS LEG #4 OF A LONG DAY OF CATCH-UP DUE TO DELAYS AND WX; SO WE WERE TIRED. I THOUGHT I HAD THE RWY 28L LOC FREQ SELECTED; BUT THE COPLT HAD THE RWY 28R LOC SET UP ON HIS SIDE; SO HE WAS PREOCCUPIED RECONFIGURING THE RADIOS AT A CRITICAL TIME. AS A LESSON; I WILL BRIEF EVERY APCH FROM NOW ON. THAT WAY; WE CAN CONCENTRATE ON ATC; TFC ETC; AND HAVE TIME TO PERFORM OTHER TASKS AS NECESSARY.