ALT EXCURSION DURING A NIGHT OP BY A B757 FLT CREW OUTSIDE THE LOM FOR AN APCH TO RWY 9L AT PBI; FL.

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THIS SIT INVOLVED SEVERAL CONTRIBUTING FACTORS: FLT SPOILERS WERE ON MAINT ORDER WHICH REDUCED DSCNT AND DECELERATION RATES. IT RESTR NORMAL AIRSPDS AND ELIMINATED AUTO SPD BRAKE DEPLOYMENT ON LNDG. THE APU WAS INOP. THE WX WAS BAD; IT WAS LATE AT NIGHT AFTER A LONG DAY. APCH CTLR WAS VERY BUSY WORKING MORE THAN ONE ARPT; SEVERAL ACR'S; AND A SMALL ACFT DOING PRACTICE APCHS AND REQUESTING MISSED APCH INSTRUCTIONS AND PRACTICE HOLDING INSTRUCTIONS. WE WERE BEING VECTORED FOR A RWY 9L ILS APCH TO PBI. ABOUT 20 MI FROM THE ARPT; WE WERE AT 7000 FT AND TOLD TO MAINTAIN 220 KTS FOR SEQUENCE AND TO RPT WHEN SLOWING. WE WERE THEN CLRED TO 3000 FT AND ALMOST IMMEDIATELY CLRED TO 2000 FT (THE CTLR SEEMED TO BE A LITTLE LATE IN CLRING US DOWN). USUALLY; THIS IS NOT A CONCERN; BUT WITH THE DEGRADED FLT SPOILERS; I WAS WORRIED ABOUT BEING ABLE TO GO DOWN AND SLOW DOWN IN TIME TO SET UP FOR THE APCH. ABOUT 5 MI FROM THE FAF; I STARTED TO SLOW. ABOUT 3 MI FROM THE FAF AND PASSING 2500 FT; WE RECEIVED AND READ BACK WHAT WE THOUGHT WAS A CLRNC TO DSND TO 1200 FT. WE WERE THEN CLRED TO A 120 DEG HDG TO INTERCEPT FINAL AND CLRED FOR THE APCH. 2 MI OUTSIDE THE FAF; I TURNED TO INTERCEPT FINAL; WAS CONFIGURING FOR LNDG; RUNNING THE LNDG CHKLIST AND CONTINUED TO DSND DOWN TO 1200 FT. THE MINIMUM DSCNT ALT INBOUND TO THE FAF IS 2000 FT. WHEN I CROSSED THE FAF; THINGS HAD CALMED DOWN AND I REALIZED I SHOULD HAVE BEEN AT 2000 FT; NOT 1200 FT. THE FIELD WAS IN SIGHT SO I CONTINUED AT 1200 FT TO INTERCEPT THE GS AND LAND UNEVENTFULLY. ALL COMS WERE VERY RUSHED AND DIFFICULT DUE TO FREQ CONGESTION AND HIGH COCKPIT WORKLOAD. AFTER TALKING THIS SIT OVER; NEITHER THE CAPT NOR I WERE ABLE TO DETERMINE IF; IN FACT; WE WERE EVER CLRED TO 1200 FT EVEN THOUGH WE READ THE CLRNC BACK. EVEN IF WE WERE; WE SHOULD NOT HAVE ACCEPTED IT BECAUSE THE APCH PLATE RESTRICTS THE DSCNT TO 2000 FT. THE CTLR NEVER SAID ANYTHING TO US ABOUT OUR ALT SO HE WAS EITHER TOO BUSY TO NOTICE OR WAS NOT CONCERNED. I INTEND TO CHANGE A FEW HABIT PATTERNS WHEN FLYING BOTH VISUAL AND INST APCHS SO AS TO PREVENT THIS FROM HAPPENING AGAIN.