B757-200 CREW DSNDED BELOW THE GS ON THE CHARTER VISUAL APCH TO SFO RWY 28R.

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THE FO WAS FLYING THE VISUAL APCH TO RWY 28R IN SAN FRANCISCO UNDER VFR CONDITIONS. THE PLANE WAS ON AUTOPLT IN VNAV MODE. WE HAD MADE ALL OF THE ALT RESTRS UP TO AND INCLUDING THE LAST RESTR. WE THEN DSNDED INADVERTENTLY BELOW THE GS MOMENTARILY ON FINAL. THIS WAS CAUSED BY SEVERAL DISTRS. TWR INFORMED US OF 2 ACFT IN THE VICINITY -- 1 AHEAD AND 1 OFF TO OUR L ON A PARALLEL APCH TO RWY 28L AND ASKED US TO SLOW TO 150 KTS. AT THE SAME TIME; THE FO THEN REQUESTED LOWERING THE GEAR SO THAT HE COULD EXTEND THE FLAPS FURTHER AND SLOW TO 150 KITS. WHILE ADJUSTING THE SPD; HE SELECTED THE FLT LEVEL CHANGE MODE WHICH ALLOWED US TO DSND THROUGH THE GS. WE THEN RECEIVED THE GS WARNING AND TWR ADVISED US OF OUR LOW ALT. THE FO REINTERCEPTED THE GS AND FLEW A STABILIZED APCH AND LNDG. I THINK WE SHOULD HAVE EMPHASIZED MORE DURING OUR BRIEFINGS THAT THE PF CONCENTRATE ON FLYING WHILE THE PNF SHOULD HANDLE DISTRS. SUPPLEMENTAL INFO FROM ACN 541036: WE WERE CLRED FOR THE VISUAL RWY 28R AT SFO. I FLEW THE APCH AS PUBLISHED. AFTER FIX XXXXX WE DSNDED DOWN TO 900 FT MSL AND THE TWR ADVISED US OF OUR ALT. WE ACKNOWLEDGED AND CLBED TO 1000 FT AND INTERCEPTED THE GS AND FLEW A STABILIZED APCH AND LNDG. I MADE THE OTHER RESTR AT XXXXX; BUT WENT BELOW GS MOMENTARILY AFTER THAT. THIS WAS DUE TO LOOKING FOR TFC; SLOWING TO 150 KTS ASSIGNED AND LOWERING THE GEAR AT THE SAME TIME; CAUSING ME TO BE DISTR MOMENTARILY AND LEAVING THE THROTTLES AT IDLE WITH THE AUTOPLT ENGAGED; CAUSING THE ACFT TO DSND THROUGH THE GS. AFTER REVIEWING THE APCH; I DON'T BELIEVE I WAS IN VIOLATION OF ANYTHING; BUT JUST WANT TO MAKE SURE; BECAUSE THE TWR HAD GIVEN US A RADIO CALL REFING OUR ALT.