FLC OF WDB ACCEPTED A QUIET BRIDGE APCH AT SFO AND ENCOUNTERED IMC ON DSCNT LOSING SIGHT OF TFC BEHIND AND BELOW THAT CAME IN CLOSE PROX.

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ATIS B CHANGED TO ATIS C ABOUT 2-3 MINS PRIOR TO LNDG. BAY APCH TRIED TO PERSUADE US TO ACCEPT A QUIET BRIDGE VISUAL APCH TO RWY 28R ABOUT 6-8 MINS BEFORE LNDG WITH A BROKEN CLOUD CONDITION ON THE ATIS. DUE TO THE CLOUD COVER; ALTHOUGH WE COULD SEE THE RWYS; I WAS NOT SURE WE COULD FOLLOW THE ALTS ON THE VISUAL APCH OR STAY VFR; SO I DID NOT ACCEPT THE VISUAL UNTIL 3-4 MINS OUT. APCH GAVE US TFC AT 3 O'CLOCK; UNKNOWN ALT; OUTSIDE THE MARKER; AND ALSO AT 7 O'CLOCK FOR 28L. HE WOULD STAY CLR OF US. WE WERE LOOKING FOR TFC AND TRANSITING FROM SFO 090 DEG RADIAL TO THE SFO 28R ILS LOC; AND ABOUT 1400 FT ENTERED A SCATTERED LAYER. I HAD NOT REALIZED WE WOULD PENETRATE. I COULD SEE SIDEWAYS AND DOWN; BUT FROM 1400 TO 800 FT MSL; I COULD NOT SEE THE RWYS AND WE WERE CENTERED ON THE 28R LOC. JUST PRIOR TO ENTERING THE CLOUD; I SAW OUR 7 O'CLOCK; ABOUT 8 O'CLOCK; 100 FT OR SO BEHIND OUR TAIL. OUR TCASII SAID THE MLG WAS 200 FT ABOVE US. AFTER WE DROPPED OUR GEAR AND CONTACTED THE TWR; I LOOKED BACK AND COULD NO LONGER SEE THE MLG ON THE 5 MI TCASII SCALE; THE PRESENTATION INDICATED THE MLG WAS ABOUT 1/8' BEHIND OUR ACFT CROSS ON THE SCOPE AND 100 FT ABOVE US. NOT SURPRISINGLY; WE GOT AN RA TO DSND MORE RAPIDLY AT 1200 FT MSL; WHICH WE DID DO. DURING THE APCH WE FLEW ASSIGNED SPDS TO THE MARKER; AND WE DID GO BELOW THE GS INSIDE THE MARKER DUE TO THE RA INSTRUCTION. I INITIALLY CONSIDERED A MISSED APCH WHEN WE WENT IFR; BUT DUE TO THE PROX OF THE MLG; I FEARED EXECUTING A GAR COULD HAVE RESULTED IN A COLLISION. I DON'T SEE HOW THE MLG COULD HAVE KEPT US IN SIGHT. I ASSUME PENETRATING CLOUD WAS AS MUCH A SURPRISE TO THEM AS IT WAS TO US. I SPOKE TO THE BAY APCH SUPVR ON DUTY; AND SAID WE RELY UPON AND SHOULD BE ASSURED OF A REASONABLY ACCURATE TERMINAL OBSERVATION. HIS RESPONSE WAS BAY APCH DOES NOT MAKE THE OBSERVATION. APCH CTL CANNOT SEE OUT ON FINAL APCH. I UNDERSTAND HIS COMMENTS; BUT SOMEONE SHOULD BE RESPONSIBLE FOR THE SAFETY ASPECTS OF THE OBSERVATION. I REMINDED HIM I DID NOT HAVE TO ACCEPT A VISUAL EVEN IN VERY GOOD WX AND WOULD BE MUCH MORE CAUTIOUS IN THE FUTURE ABOUT BELIEVING THEM. HE ACKNOWLEDGED WE HAD A VERY DANGEROUS POTENTIAL AND SAID IT WOULD BE DISCUSSED IN A STAFF MEETING. MY COMPANY'S MGR ON DUTY WAS DISTURBED ABOUT THE INCIDENT AND STATED IT WOULD BE REVIEWED WITH SFO APCH. A LAST COMMENT - THE RADIO FREQ CONGESTION WAS SO BAD WE COULD NOT MAKE A RADIO XMISSION DURING THE PERIOD FROM 1400 FT DOWN TO 700 FT.