A B727 CREW; ON APCH TO SAN; DEPARTED ASSIGNED ALT PREMATURELY; PRIOR TO XING ASSIGNED RADIAL.

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IT WAS A BEAUTIFUL CLR DAY IN SAN. IT WAS MY SECOND APCH INTO RWY 27 AT SAN WITHIN ABOUT THE LAST 3 YRS. THE DAY PREVIOUS; ATC HANDLED OUR FLT COMPLETELY DIFFERENTLY. WE WERE FLYING INTO SAN FROM THE WNW. WE WERE ON ABOUT THE 300 DEG RADIAL OF MZB VOR CLRED TO MZB VOR DIRECT SAN. WE COULD SEE THE ARPT FROM MANY MI OUT; BUT DID NOT YET RPT IT IN SIGHT. A FEW MI BEFORE MZB VOR; SOCAL APCH TURNED US 10 DEGS L TO AN ASSIGNED HDG OF 110 DEGS RADAR VECTORS TO THE VISUAL APCH TO RWY 27 SAN. IN THE VICINITY OF MZB VOR; ATC ASKED US IF WE HAD THE ARPT IN SIGHT. WE CALLED THE ARPT IN SIGHT. SOCAL APCH CLRED US FOR THE VISUAL APCH TO RWY 27; AND TO CROSS THE MZB 084 DEG RADIAL AT 5000 FT MSL. (THIS I ASSUMED WAS TO KEEP US IN THE CLASS B AIRSPACE.) AT THIS POINT WE WERE EITHER DSNDING TO 5000 FT OR LEVEL AT 5000 FT MSL. WE WERE XING JUST N OF THE MZB VOR. MY NAV INSTS WERE SET UP FOR THE LOC TO RWY 27 (BECAUSE WE WERE ON A RADAR VECTOR). I HAD THE FO SET UP THE MZB 084 DEG RADIAL. THEN; I ASSUMED ON OUR CURRENT TRACK AND HDG WE SHOULD CROSS THE MZB VOR 084 DEG RADIAL CLOSE TO THE VOR; SO; AT ABOUT 6 MI E OF MZB VOR; I STARTED DOWN FROM 5000 FT MSL (ASSUMING I ALREADY PASSED THE MZB 084 DEG RADIAL) AND COMPLETED THE R BASE VISUAL TO RWY 27. FROM THE MOMENT WE WERE CLRED FOR THE VISUAL; THE COCKPIT WORKLOAD WAS VERY BUSY. PROB: I MAY HAVE DSNDED BELOW 5000 FT MSL BEFORE XING THE MZB 084 DEG RADIAL. CONTRIBUTING FACTORS: 1) IT WAS DAWN AND I WAS FEELING FATIGUED (HITTING A SINKER). 2) I ASSUMED MY ASSIGNED HDG WOULD TRACK ACROSS THE MZB 084 DEG RADIAL. 3) THE WX WAS SO NICE. IT MAY HAVE MADE ME COMPLACENT. 4) ATC HANDLED US DIFFERENTLY THE DAY BEFORE. 5) MY FLT INSTS WERE SET UP FOR THE RWY; NOT THE MZB VOR. HOW TO AVOID IN THE FUTURE: 1) WHEN GIVEN A CLRNC TO CROSS A CERTAIN RADIAL/FIX AT A CERTAIN ALT; NEVER ASSUME IT IS COMPLIED WITH. ALWAYS SET UP THE NAV RADIOS TO VERIFY COMPLIANCE. 2) IN VFR WX ON A VISUAL APCH; ALWAYS SET UP SOMEONE'S NAV RADIOS TO NAV IN AND THROUGH THE CLASS B AIRSPACE. 3) EXTEND FURTHER DOWNWIND TO ALLOW MORE TIME FOR THE APCH.