CLOSE PROX ACR MLG RETURNING TO SJC BECAUSE OF PRESSURIZATION PROBLEMS AND COMMUTER LTT INBOUND TO PARALLEL RWY.

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AFTER TKOF FROM SJC WE HAD EXPERIENCED A PROBLEM WITH A DOOR SEAL AND IT APPEARED WE WOULD NOT BE ABLE TO PROPERLY PRESSURIZE THE ACFT. WE WERE ON THE LOUPE 8 DEP (SJC) AT 12000 FT (120 DEG HDG) WHEN WE DETERMINED THAT WE WERE GOING TO HAVE TO RETURN TO THE ARPT. WE ADVISED BAY DEP OF OUR SITUATION AND REQUESTED VECTORS FOR A RETURN. WE TOLD THEM THAT WE DID NOT HAVE AN EMER AND WE WOULD NOT NEED EQUIP STANDING BY FOR OUR LNDG. DEP ADVISED US TO CONTINUE OUR PRESENT HDG AND EXPECT VECTORS FOR RWY 30L AT SJC. WE WERE THEN HANDED OFF TO BAY APCH AND RECEIVED A CLRNC TO DSND TO 4000 FT AND A R TURN TO 230 DEGS (APPROX BASE LEG TURN TO PUT US ONTO THE 30L LOC). AS WE DSNDED THROUGH 7000 WE WERE TOLD TO 'MAINTAIN 7000.' AT THIS TIME I NOTICED A TWIN ENG TURBOPROP (COMMUTER) IN A WINGS UP STEEP R TURN; PASSING THROUGH OUR 12 O'CLOCK POS. HE APPEARED TO BE SLIGHTLY LOWER THAN US; POSSIBLY ABOUT 200 FT. WE ROLLED INTO A R TURN TO ALLOW ADDITIONAL CLRNC AND; AT THAT POINT; I LOST VISUAL CONTACT WITH THE TARGET. I WOULD ESTIMATE THAT WE NEVER CAME CLOSER THAN 1000 FT LATERALLY; ALTHOUGH THE CAPT THOUGHT WE MAY HAVE BEEN CLOSER THAN THAT. JUST AFTER WE HAD SPOTTED THE TFC; ATC ADVISED US OF THE TARGET. IT APPEARS THAT HE HAD STARTED EVASIVE ACTION BEFORE ATC SPOTTED THE CONFLICT AND BEFORE WE ACQUIRED VISUAL CONTACT. ALTHOUGH I'M NOT SURE OF ALL OF THE DETAILS; I ASSUMED THAT THE 'OTHER GUY' WAS ON THE LOC FOR 30L AT SJC. HE WAS IN THE RIGHT PLACE AND WAS ON THE SAME FREQ WE WERE WORKING WITH BAY APCH. I DID HEAR HIM TELL APCH THAT HE HAD A TCASII (OUR ACFT WAS NOT TCASII EQUIPPED). IT APPEARS THAT THE ATC CTLR HAD TURNED US RIGHT INTO THE OTHER ACFT'S PATH. BY WAY OF EXPLANATION TO THE OTHER PLT; HE SAID HE 'HAD AN MLG WITH AN EMER.' WE HAD NOT DECLARED AN EMER NOR HAD WE REQUESTED PRIORITY HANDLING. IT LOOKS LIKE THE CTLR HAD OVERREACTED TO THE SITUATION; POSSIBLY BECAUSE THE PREVIOUS POS HAD NOT GIVEN HIM ALL OF THE INFO. AT THE TIME OF THE INCIDENT THE COCKPIT WORKLOAD WAS AT A HIGH LEVEL. AS THE PNF; I WAS TRYING TO COMPLETE THE DSCNT CHKLIST; THE IN RANGE CHKLIST; AND JUGGLE RADIO COMS WITH BOTH THE COMPANY AND ATC. IN EFFECT THEN; 1 PLT WAS 'OUT OF THE LOOP' WHEN IT CAME TO MAINTAINING A VISUAL SCAN FOR TFC. I DON'T KNOW IF THE OTHER CREW HAD ESTABLISHED VISUAL CONTACT WITH US PRIOR TO THEIR TCASII WARNING; BUT THEIR TIMELY EVASIVE ACTION PREVENTED THE SITUATION FROM POSSIBLY BECOMING MUCH WORSE.