ACR X LOS OF ACFT CTL FROM B757 WAKE TURB WHILE ON VISUAL APCH.

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ACR X VECTORED ON DOWNWIND AT 180 K; CLRED TO 4000 FT BY APCH CTL. IN THE DSCNT WE WERE DIRECTED TO EXPEDITE THROUGH 5000 FT FOR TFC XING L TO R AT 11:30; 6000 FT; A B-757. WE CALLED THE TFC INSIGHT AS WE VERIFIED ALT AND POS ON THE TCASII APCH DIRECTED US TO MAINTAIN VISUAL SEPARATION AND ADVISED WE WOULD FOLLOW THE 757 TO RWY 26R. THE 757 WAS PAST OUR 12:00 POS AND ON A DOG-LEG TO THE FINAL APCH COURSE AS WE WERE VECTORED TO HDG 180 AND SLOWED TO 170 KTS. THE PRECEDING 757 SHOWED 2 MI FROM OUR ACFT WHEN APCH TURNED US R TO 220 AND CLRED US FOR THE VISUAL APCH. I IMMEDIATELY ADVISED APCH THAT WE WERE TOO CLOSE AND ASKED FOR TFC BEHIND US SO THAT WE COULD SLOW DOWN. APCH DID NOT COMMENT ON TFC BUT SAID WE COULD TURN TO 200 DEG FOR INTERCEPT AND SLOW TO 160 KTS. WE CROSSED THE LOC HDG 200 DEG AT 160 KTS BUT WERE FORCED TO CORRECT BACK TO COURSE DUE TO PARALLEL TFC ON THE VISUAL APCH TO RWY 27L. WE HAD STARTED DSCNT; HOLDING ABOUT 1 1/2 DOTS HIGH ON THE GS INTENTIONALLY; WHEN WE ENCOUNTERED THE 757'S WAKE TURB. THE FO; WHO WAS FLYING THE ACFT; RESPONDED TO THE BRIEF ENCOUNTER WITH TIMELY ROLL CTL INPUTS; ADDED PWR; AND LOWERED THE NOSE TO MAINTAIN AIRSPD AND RECOVER TO STABLE FLT. MAX UNCOMMANDED BANK WAS 15 TO 20 DEG AND AIRSPD LOSS WAS NOTED BUT ITS EXTENT WAS NOT NOTED. WE CONTINUED STRAIGHT AHEAD UNDER TWR DIRECTION AND WERE RETURNED TO APCH CTL FOR VECTORS IN THE PATTERN TO AN UNEVENTFUL LNDG. DURING THIS APCH; WE WERE CLOSE TO 1 3/4 MI TO THE PRECEDING TFC (DURING THE GAR) AND NEVER SPACED MORE THAN 2 1/2 MI. THIS SEPARATION WAS INADEQUATE FOR FOLLOWING COMMUTER TFC AND TOTALLY UNACCEPTABLE BEHIND A B757 WITH ITS SPECIAL WAKE TURB HAZARD. I FEEL THAT THE CTLR SET UP A SIT THAT PRODUCED INADEQUATE SEPARATION IN HIS DESIRE TO KEEP TFC MOVING EFFICIENTLY. WE REALIZED AS THE SIT DEVELOPED THAT WE WERE TECHNICALLY RESPONSIBLE FOR SEPARATION ONCE WE CALLED THE TFC IN SIGHT ON DOWNWIND BUT WERE BOXED IN BY THE CONSTRAINTS OF FOLLOWING ASSIGNED VECTORS AND SPDS FOR SEPARATION WITH OTHER TFC; FREQ CONGESTION THAT PREVENTED TIMELY COM WITH CTLRS; AND NOT KNOWING THE SEPARATION `GAME PLAN' UNTIL BEING TURNED TO FINAL WHERE WE EXERCISE OUR ONLY AVAILABLE OPTION (THE GAR). THE TCASII SYS WAS INVALUABLE IN PROVIDING SPECIFIC SEPARATION DISTANCE MEASUREMENTS TO US TO SUPPLEMENT THE UNCALIBRATED EYE-BALL. EVEN WITH THE NUMEROUS TARGETS PRESENTED DURING HIGH DENSITY PARALLEL APCHS WE WERE ABLE TO SEE THE SIT DEVELOPING AND WERE READY WHEN SAFE SEPARATION WAS LOST. ONLY THE WAKE TURB FROM THE 757 DSNDING AHEAD OF US ON WHAT LATER BECAME OUR FLT PATH PROVED UNPREDICTABLE THOUGH NOT UNEXPECTED. MY FEELING IS THAT A REQUEST FOR 4 TO 5 MI BEHIND A B757 IS IN ORDER ON ALL APCHS AND DEPS AND SHOULD BE MANDATORY.