MULTIPLE RWY OP PARALLEL RWY APCHS. ACR X HAD LTSS FROM WDB ON VISUAL APCH TO PARALLEL RWY.

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ATC INDICATED VISUAL APCHS. DETERIORATING WX (SCUD LAYER COMING THROUGH THE 'GAP') CAUSED A TEMPORARY CONDITION OF 1000 FT BROKEN; SO ACFT WERE VECTORED AND SEQUENCED FOR THE ILS RWY 28R. WX THEN IMPROVED SUFFICIENTLY; SO THAT THE ARPT WAS VISIBLE (SCUD LAYER HAD MOVED). WE RPTED THE FIELD AND WERE CLRED FOR THE QUIET BRIDGE VISUAL RWY 28R. WE WERE TOLD TO MAINTAIN VISUAL ON TFC FOR RWY 28L; WHICH WAS A WDB. THE WDB WAS ON A CONSTANT BEARING AND I ASKED; 'WHO'S GOT THE LEAD?' I RECEIVED NO RESPONSE. WE WERE THEN TOLD TO FLY 160 KTS (PRIOR TO BRIJJ). WE FLEW SLIGHTLY FASTER BECAUSE THE WDB APPEARED TO BE CONVERGING ON US; AND WE STAYED R OF THE RWY 28R CTRLINE. AT ABOUT 2 NM; TWR TOLD US TO 'CONTINUE' BECAUSE TFC IN FRONT OF US WAS JUST TOUCHING DOWN. WE SLOWED TO APCH SPD AND THE WDB PASSED US APPROX 1/2 NM FROM TOUCHDOWN; AS WE WERE DSNDING THROUGH 300 FT. THIS WAS EXTREMELY DISCONCERTING; PARTICULARLY WITH THE POSSIBILITY OF THE WDB'S WAKE TURB HITTING US AT LOW ALT. THE 7110 (AIR TFC CTLR'S HANDBOOK) PROHIBITS A LARGER ACFT FROM PASSING A SMALLER ACFT. I CANNOT ASCERTAIN WHETHER BAY TRACON IS DISREGARDING THIS REQUIREMENT OR IF THE RELATIVE MOTION BTWN 2 PARALLEL ACFT IS SO DIFFICULT TO ADEQUATELY DETERMINE THAT THIS SEPARATION CANNOT BE ACCURATELY MAINTAINED.