MLG MADE GAR WHEN SPACING WAS LOST WITH THE PRECEDING ACFT.

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WHILE BEING VECTORED TO FINAL APCH AT DEN; A RWY CHANGE FROM 17L AND R TO RWY 26L AND R OCCURRED. APCH CTL DID A NICE JOB OF KEEPING THINGS IN ORDER USING A MIN OF SEPARATION BTWN ACFT. WE WERE GIVEN THE POS OF OUR TFC TO FOLLOW AND CALLED HIM IN SIGHT. WE ACCEPTED THE 'VISUAL APCH' CLRNC SEEING A 3 NM SEPARATION FROM OUR TFC ON OUR TCASII SYS. WE ANTICIPATED OUR TFC MAINTAINING THE REQUESTED '170 KTS TO THE MARKER;' HOWEVER WE BEGAN TO NOTICE A VERY GRADUAL DECELERATION. WE WERE THEN DISTR BY THE TCASII SYS ISSUING RAS ON THE PARALLEL APCH TFC. IN WATCHING THE PARALLEL TFC; WE DID NOT NOTICE THE DECELERATION OF OUR TFC TO FOLLOW UNTIL SEPARATION HAD DECREASED TO APPROX 2 1/2 NM. WE THEN KNEW THE TIMING OF OUR TFC'S EXIT FROM THE RWY WOULD BE CRITICAL... SO DID THE TWR. THE TWR REQUESTED THE TFC TO EXIT AT THE HIGH SPD EXIT. THE TFC SAID THEY WERE UNABLE AND PROCEEDED TO THE NEXT EXIT. OUR ALT (AGL) ON FINAL WAS LESS THAN 500 FT WHEN THE TFC GOT TO THE NEXT EXIT. CHANCES ARE WE WOULD HAVE TOUCHED DOWN AS THE TFC CLRED; HOWEVER; AND RIGHTFULLY SO; THE TWR TOOK THE MORE CONSERVATIVE OPTION AND DIRECTED US TO 'GAR.' AN UNEVENTFUL GAR; APCH AND LNDG FOLLOWED. WE RECOGNIZE IT IS OUR RESPONSIBILITY TO MAINTAIN ADEQUATE SEPARATION FROM TFC DURING VISUAL APCHS. MANY FACTORS WERE COMING TOGETHER ON THIS VISUAL APCH THAT REQUIRED ULTIMATE ATTN TO DETAIL. AT THE END OF A LONG DAY; WE ALLOWED OURSELVES TO BE DISTR BY THE TCASII RAS ON THE PARALLEL TFC TO RWY 26R. ALTHOUGH THE TCASII WAS EXTREMELY USEFUL IN RECOGNIZING SEPARATION ON OUR TFC; IT ULTIMATELY LEAD TO A LOSS OF SEPARATION NECESSARY TO LAND OUT OF THE VISUAL APCH. AGAIN; LOSS OF ADEQUATE SEPARATION WAS OUR RESPONSIBILITY. BUT TCASII RA'S ARE SERIOUS OCCURRENCES THAT REQUIRE ATTN. HOWEVER; IT SEEMS WE MAY HAVE GIVEN IT TOO MUCH ATTN AND NOT ENOUGH ON OUR TFC AHEAD.