A COMMUTER FLC CONTINUED TO THE DEST AFTER HAVING RECEIVED 2 BLEED AIR FAIL ANNUNCIATORS INFLT. THE CAPT TROUBLESHOT THE PROB AND INCORRECTLY CONCLUDED THAT THE WARNING WAS NOT HARMFUL. 1 BLEED AIR SWITCH WAS KEPT IN THE ON POS TO MAINTAIN CABIN PRESSURIZATION.

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FLT DEPARTED BOSTON LOGAN ARPT FOR PORTLAND; MA; ON AN IFR FLT PLAN IN VFR CONDITIONS. THE R BLEED AIR FAIL ANNUNCIATOR SYS WAS DEFERRED PER ACFT MEL; BECAUSE THE R BLEED AIR FAIL ANNUNCIATOR HAD ILLUMINATED INTERMITTENTLY ON THE GND. THE MAINT DEPT AND THE CAPT DECIDED THERE WAS AN ERROR IN THE ANNUNCIATOR SYS SINCE THE BLEED AIR SYS WAS WORKING PROPERLY AND THE ANNUNCIATOR ONLY ILLUMINATED INTERMITTENTLY ON THE GND. DURING THIS FLT TO PORTLAND BOTH BLEED AIR FAIL ANNUNCIATORS ILLUMINATED DURING FLT. THE FO WAS FLYING THE ACFT AND HE CALLED FOR IDENT OF THE RED MASTER WARNING LIGHTS. THE CAPT IDENTED THE NATURE OF THE EMER AND PERFORMED THE EMER MEMORY ITEM -- TO TURN THE BLEED AIR VALVE TO THE INST AND ENVIRONMENTAL OFF POS. THE FO CALLED FOR THE EMER CHKLIST AND THE CAPT READ AND PERFORMED THE 2 CHKLIST ITEMS -- TO TURN THE VALVE OFF AND MONITOR THE ENG INSTS. SINCE BOTH BLEED AIR VALVES WERE OFF; THE ACFT BEGAN TO DEPRESSURIZE. THE CAPT ELECTED TO CONTINUE TO THE PORTLAND ARPT; BECAUSE THE FLT WAS ALREADY 1/3 OF THE WAY TO PORTLAND. THE CAPT ALSO DECIDED TO TROUBLESHOOT THE BLEED AIR SYS BY TURNING EACH BLEED AIR VALVE INDIVIDUALLY OFF TO TRY TO DETERMINE WHETHER THE DUAL BLEED AIR FAIL ANNUNCIATORS WERE LEGITIMATE OR WHETHER THE ANNUNCIATOR SYS WAS FAILING. IN THE PROCESS OF TROUBLESHOOTING THE SYS BOTH BLEED AIR FAIL ANNUNCIATOR LIGHTS WENT OUT. IT APPEARED AS IF THE SYS WAS WORKING PROPERLY EVEN THOUGH BOTH LIGHTS HAD ILLUMINATED MOMENTARILY. THE CAPT CHOSE TO LEAVE THE L BLEED AIR VALVE IN THE ON POS SINCE THE MEL FOR THE R BLEED AIR FAIL ANNUNCIATOR SYS STIPULATED THAT THE R BLEED AIR VALVE MUST REMAIN IN THE INST AND ENVIRONMENTAL OFF POS. BY LEAVING THE L BLEED AIR VALVE IN THE ON POS THE ACFT BEGAN TO PRESSURIZE AGAIN MAKING THE TRIP MORE COMFORTABLE FOR THE PAX. AFTER ARRIVING IN PORTLAND THE CAPT SPOKE WITH THE MAINT DEPT AND THEY ASKED THE CREW TO FERRY THE ACFT TO THE MAINT BASE. THE MECHS FOUND A SMALL HOLE IN THE EVA TUBING WHICH IS THE REASON THE ANNUNCIATORS BOTH ILLUMINATED DURING FLT. IN RETROSPECT; THE CAPT UNDERESTIMATED THE NATURE OF THE FAILURE; BECAUSE THE R BLEED AIR FAIL ANNUNCIATOR SYS HAS ASSUMED TO BE INOP. IN THE FUTURE; I THINK I WILL APCH MECHANICAL IRREGULARITIES WITH MORE SKEPTICISM AND NOT ASSUME THAT BECAUSE AN ANNUNCIATOR SYS HAS BEEN DEFERRED THAT THERE ISN'T SOMETHING MORE EXTENSIVE GOING ON WITH THE SYS ITSELF. WE FLY ACFT ALL THE TIME THAT HAVE DEFERRED ITEMS FOR WHICH WE HAVE LITTLE HISTORY ABOUT EXCEPT WHAT THE PREVIOUS CREW OR MAINT TELLS US. I ALSO LEARNED FROM THIS SIT THAT THE ANNUNCIATOR SYS WAS ALERTING US NOT TO A COMPLETE FAILURE OF THE BLEED AIR SYS (WHEN THE EVA TUBING MELTS) BUT TO AN IMMINENT FAILURE OF THE SYS IF WE HAD CONTINUED TO FLY THE ACFT WITH THE BLEED AIR VALVES ON.