AN ACR FLC RECEIVED A GPWS TERRAIN AND PULLUP WARNING IN IMC; WHILE BEING VECTORED OUTSIDE CLASS B AIRSPACE. THE FLC RESPONDED TO THE ALERT AND NOTIFIED ATC. AS THEY WERE VECTORED INBOUND; AGAIN; THE FLC INADVERTENTLY DSNDED BELOW THE CLRNC ALT. CTLED FLT TOWARD TERRAIN. ALTDEV ALT EXCURSION.

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WE WERE BEING VECTORED FOR THE RWY 27 LOC APCH. OUR POS WAS THE STANDARD DOWNWIND VECTOR AT 5000 FT; N OF AND PARALLEL TO THE 272 DEG RADIAL INBOUND COURSE. WE HAD DEVIATED ONCE TO AVOID A RAIN SHOWER AND THERE WERE MORE FURTHER E. APPROX 15-16 MI E OF THE ARPT WE WERE GIVEN A R TURN TO A 180 DEG HDG (BASE TURN). SHORTLY THEREAFTER ATC TURNED US BACK TO 090 DEGS. HE STATED THIS WAS A VECTOR OUTSIDE CLASS B AIRSPACE FOR INBOUND EMER TFC INTO NORTH ISLAND NAS. I PUT BOING LMM (NAVAID FOR THE MSA) IN THE FIX PAGE AND SELECTED A 25 NM ARC. THIS GAVE US A PICTURE OF THE MSA LIMITS TO THE E (5200 FT). I ALSO CHKED THE TERRAIN ON THE DEP AREA CHART AND NOTED THAT THE TERRAIN DID IN FACT RISE FURTHER E; BUT DIDN'T APPEAR TO BE DANGEROUSLY HIGH. I HAVE FLOWN INTO SAN A NUMBER OF TIMES IN VFR CONDITIONS AND FELT WE WERE SAFE INSIDE THE MSA AT 5000 FT. SHORTLY BEFORE XING THE 25 NM ARC ATC TURNED US TO S TO 180 DEGS. WHILE ON THAT HDG THE GPWS TERRAIN-PULLUP ALERT SOUNDED. I INITIATED A HAND FLOWN CLB AND TOLD THE FO TO RADIO ATC THAT WE WERE CLBING DUE TO THE TERRAIN ALERT. ATC AT FIRST SEEMED CONFUSED ON OUR TFC CALL; NO DOUBT HE WAS ALSO WORKING THE EMER INTO NORTH ISLAND. HE STATED WE WERE SAFE AT 5000 FT AND TURNED US TOWARD THE ARPT; WITH A 240 DEG INTERCEPT HEADING TO THE LOC. BY THIS TIME I HAD CLBED TO 5500 FT AND THE TERRAIN WARNING HAD CEASED. TO SAY THAT I WAS AGITATED BY THE ALERT WOULD BE AN UNDERSTATEMENT. THE ACR CFIT ACCIDENT IN COLUMBIA WAS CERTAINLY ON MY MIND. I STARTED A DSCNT BACK TO 5000 FT. AT THIS POINT I WAS DISTRACTED (BY WHAT I STILL CAN'T RECALL). THE NEXT XMISSION FROM ATC STATED WE WERE ONLY CLRED TO 5000 FT. WE WERE DSNDING THROUGH 4500 FT. I IMMEDIATELY CLBED BACK TO 5000 FT. AFTER GETTING BACK ON ALT THE REST OF THE APCH WAS FLOWN UNEVENTFULLY. CONTRIBUTING FACTORS: THIS WAS MY FIRST TRIP AFTER A 1 MONTH VACATION. THE DAY HAD BEEN SOMEWHAT LENGTHENED BY WX DELAYS. THERE WERE RAIN SHOWERS AND TSTMS IN THE AREA. THE ONLY CONCLUSION I CAN COME TO IS THAT I WAS TIRED AND BECAME DISTRACTED BY THE 'TERRAIN ALERT' AND FAILED TO MAINTAIN MY SITUATIONAL AWARENESS. AS MY FO SAID; THIS APCH HAD ALL THE ELEMENTS FOR A LESSON IN CFIT EXPOSURE.