A B737-300 FLC EXPERIENCES AN ENCOUNTER WITH WAKE TURB WHEN ON A VISUAL APCH TO MCO. THE PRECEDING ACFT WAS A B757 4 PT 5 MI AHEAD. ATC HAD ISSUED A SPD RESTR TOO LOW FOR THE B737 PIC TO COMPLY WITH SO WAS GIVEN AN EXTENDED VECTOR TO FOLLOW THE B757 WITH STANDARD SEPARATION. FOLLOWING THIS THE B737 HAD ANOTHER EVENT DURING FLARE AFTER THE PARALLEL RWY WAS NOT GIVEN TO HIM AS REQUESTED BY PIC.

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WE WERE BEING VECTORED XWIND; 15 NM FROM RWY. CTLR ASSIGNED A SPD OF 160 KTS TO FOLLOW A HVY B757. I WAS TOLD TO 'ADVISE IF UNABLE;' AND I REFUSED THE SPD FOR WAKE TURB CONSIDERATIONS. CTLR REPLIED WITH AN EXPECTED DELAY VECTOR ACROSS FINAL FOR SPACING. SHORTLY WE WERE TURNED TO FINAL AND ALMOST IMMEDIATELY ENCOUNTERED WAKE TURB. AUTOPLT WAS DISENGAGED AND ACFT FLOWN OFF FINAL. TCASII SHOWED PRECEDING ACFT 4 1/2 MI AHEAD AND 1100 FT LOWER. A HIGH FINAL WAS FLOWN AND WHEN WE WERE HANDED OFF TO TWR; WE REQUESTED THE PARALLEL RWY (RWY 18L). TWR REJECTED THE REQUEST BECAUSE IT WAS UNAVAILABLE. AFTER LNDG; I ASKED TWR WHY IT WAS NOT AVAILABLE AND HE RESPONDED; BECAUSE IT WOULD HAVE DELAYED THE PRECEDING ACFT XING THAT RWY TO GET TO HIS GATE. I EXPLAINED WE HAD REQUESTED IT FOR WAKE TURB CONSIDERATIONS AND HAD ENCOUNTERED WAKE TURB ON FINAL (ACTUALLY ON FINAL AND IN THE FLARE). EACH CTLR HAD STATED 'FOLLOWING HVY; USE CAUTION...' BUT THEN SLOWED ME DOWN (REDUCING MY SAFETY MARGIN) AND REFUSED ME AN ALTERNATE RWY. IT WOULD APPEAR MORE CONSIDERATION COULD HAVE BEEN APPLIED TO AVOID A POSSIBLE DANGEROUS SIT.