A B737-300 PIC HAS A POTENTIAL CONFLICT WITH AN E120 WHILE ON AN 'FMS BRIDGE VISUAL' APCH TO RWY 28R AT SFO; CA.

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NOTE: THIS IS NOT RPTED AS A NEAR MISS BECAUSE I MAINTAINED VISUAL CONTACT WITH THE OTHER ACFT AND FELT LIKE I WAS IN CTL OF THE SIT. FLT WAS CLRED FOR THE SAN FRANCISCO FMS BRIDGE VISUAL RWY 28R APCH WHILE OUTSIDE OF THE ARCHIE FIX. THE WX WAS JUST ABOVE MINIMUMS (2100 FT AND 5 MI) REQUIRED FOR THE APCH. AFTER BEING CLRED FOR THE APCH; WE WERE ADVISED TO WATCH FOR A BRASILIA THAT WOULD BE SHOOTING A VISUAL TO RWY 28L. WE MADE VISUAL CONTACT WITH THE BRASILIA; OBSERVING IT TO BE ON A BASE LEG INTERCEPT; AND WERE INSTRUCTED TO MAINTAIN VISUAL SEPARATION. THE BRASILIA WAS CLOSING ON US RAPIDLY JUST AS WE ENTERED A BROKEN CLOUD DECK; ABOUT 2 MI SHORT OF THE GAROW FIX. WE OBSERVED THE BRASILIA ON TCASII AND AS IS CUSTOMARY ON THE SFO PARALLEL APCHS; HAD SELECTED THE TCASII TO TA TO AVOID CONSTANT WARNINGS DURING THE APCH. TCASII SHOWED THE BRASILIA TO BE EXTREMELY CLOSE TO US. I BECAME CONCERNED WITH AVOIDING THE BRASILIA AS WE WERE IN AND OUT OF THE CLOUDS AND IN THE PROCESS FLEW BELOW THE 'AT OR ABOVE' 4000 FT AT THE GAROW FIX. I THINK WE WERE PROBABLY AT ABOUT 3600 FT OVER GAROW. THE COPLT AND AN ACR JUMP SEAT RIDER WERE ALSO SO CONCERNED WITH THE BRASILIA'S CLOSE PROX THAT NEITHER OF THEM NOTICED THE DSCNT BELOW THE ALT OF GAROW. THE TFC IN THE BAY AREA WAS QUITE HVY AND THE WX WAS FAR FROM CLR. WE WERE ALSO GIVEN A LARGE AIRSPD REDUCTION AT THE SAME TIME TO PICK UP MINIMUM SPACING ON A PRECEDING B757. THIS UNDOUBTEDLY CONTRIBUTED TO THE DISTR. THE CONCERNS OVER THE BRASILIA TURNED OUT TO BE WELL FOUNDED. IT APPEARED TO US THAT HE WAS SHOOTING AN APCH TO RWY 28R. THE COPLT ASKED TWR TWICE IF THE BRASILIA WAS GOING TO RWY 28L. THE BRASILIA MAINTAINED ITS R OF COURSE TRACK UNTIL IT WAS ABOUT 2 MI FROM THE RWY AND THEN DRIFTED TO THE L; ALLOWING US TO LINE UP WITH RWY 284 AT ABOUT 500 FT AGL. IN SUMMARY; I HAD AN ALTDEV OVER THE GAROW FIX; BUT FEEL IT WAS JUSTIFIED IN THE INTEREST OF SEE AND AVOID. AT NO TIME; AFTER I ACQUIRED THE BRASILIA; DID I FEEL OUR ACFT WAS IN JEOPARDY. IN THE FUTURE; I WILL NOT ACCEPT AN FMS 'VISUAL' IN MARGINAL BUT 'LEGAL' WX CONDITIONS.