FLC OF A CESSNA 560 CPR JET LOST SIGHT OF THE B737 THEY WERE INSTRUCTED TO VISUALLY FOLLOW DURING A VISUAL APCH AND ASKED TO MAKE A GAR BY TWR.

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I WAS ACTING AS PIC IN THE R SEAT; SIC WAS PF THE ACFT; BEING VECTORED FOR VISUAL APCH TO RWY 28R AT PDX. PER ATIS; WE WERE EXPECTING A MILL VISUAL TO RWY 28R. WERE TO FOLLOW A B737. APCH CTLR VECTORED THE B737 S OF THE MILL VISUAL APCH PATH FOR A STRAIGHT-IN APCH TO RWY 28R. APPROX 3 MINS EARLIER; ATC ASKED FOR A SPD OF 210 KTS. AS WE WERE PERPENDICULAR TO APCH COURSE; WE SIGHTED THE B737. ADVISED ATC; THE ACFT WAS IN SIGHT AND WERE TOLD TO FOLLOW THE B737; CLRED FOR THE VISUAL TO RWY 28R. ABOUT THIS TIME WE HAD A PRESSURIZATION PROB AND THE PAX HAD QUESTIONS ABOUT THE PROB. BOTH PLTS RESPONDED TO THE PAX. MEANWHILE THE PF WAS STILL MAINTAINING 210 KTS AND WAS FLYING THE MILL VISUAL; WHICH WAS A STRAIGHT LINE TO RWY 28R. THEN WE WERE ADVISED BY ATC TO CONTACT TWR. PF THEN STATED HE HAD LOST THE B737. SWITCHED FREQS TO TWR AND BOTH PLTS WERE LOOKING FOR THE B737. TWR THEN CALLED US AND RPTED B737 AT OUR 9 O'CLOCK. THEN REACQUIRED VISUAL AND WERE TOLD TO GAR BY ATC. COMBINATION OF FACTORS CAUSED PROB: MAINTAINING PREVIOUSLY ASSIGNED AIRSPD AFTER CLRED FOR VISUAL APCH; FLYING MILL VISUAL APCH RATHER THAN STRAIGHT-IN BEHIND THE PRECEDING ACFT. THIS WAS DUE TO A COMBINATION OF BEING TOLD TO EXPECT THAT APCH AND A PREVIOUS PATTERN OF BEING ASSIGNED THE MILL VISUAL ON MOST OF OUR ARRS; AND DISTRS FROM PAXS CAUSED US TO LOSE SIGHT OF THE PREVIOUS ACFT.