NUMEROUS MECHANICS REPLACING A BOMBARDIER DHC-8-400 ACFT MAIN CABIN DOOR SEAL ARE LATER INFORMED THE DOOR SEAL AIR RESERVOIR END HAD BLOWN OFF AT THE END SEAMS.

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DURING AN 'A' CHK; AN AVIONICS TECHNICIAN WAS USING SOME TEST EQUIP ON THE ACFT WHICH CAUSED THE MAIN CABIN DOOR SEAL TO INFLATE. NO ONE WORKING THE 'A' CHK KNEW WHAT HE WAS DOING OR THAT THE DOOR SEAL WAS INFLATED. A MECH WENT TO CLOSE AND THEN OPEN THE DOOR CAUSING THE DOOR SEAL TO TEAR. MR X AND I WERE THEN INSTRUCTED TO CHANGE THE SEAL. AFTER CHANGING THE SEAL; WE PERFORMED AN OPS CHK OF THE DOOR SEAL. NOT REALIZING THAT THE DOOR SEAL RESERVOIR WAS ALREADY FILLED BY THE TEST PERFORMED BY AVIONICS; WE BEGAN TO SVC THE RESERVOIR. THE MINUTE THAT MR Z CRACKED OPEN THE LOW PRESSURE REGULATOR WE ALL HEARD A LOUD BOOM. MOST PEOPLE WORKING IN THE AREA THOUGHT THAT THE NOISE CAME FROM THE R ENG SINCE THERE WAS AN AIRBORNE KIT HOOKED TO IT FOR DEICE BOOT CHKS. THE CREW PERFORMED OPS CHKS AND FOUND NO DISCREPANCIES WITH THE DEICE SYS; EXTINGUISHING SYS. UPON ARRIVING AT WORK ON A LATER DATE; I WAS INFORMED THAT THE END HAD BLOWN OFF OF THE DOOR SEAL RESERVOIR CAUSING STRUCTURAL DAMAGE TO THE ACFT. THE EVENT OCCURRED BECAUSE OF LACK OF COM BTWN MECHS AND AVIONICS SINCE WE WEREN'T AWARE THAT THE RESERVOIR WAS ALREADY FILLED. SUPPLEMENTAL INFO FROM ACN 780600: MYSELF AND MS Y WERE ASSIGNED TO CHANGE OUT A MAIN CABIN DOOR SEAL ON ACFT X DUE TO A TEAR RESULTING FROM MISCOM BTWN AVIONICS AND MAINT. NO ONE ON SHIFT HAD ANY EXPERIENCE CHANGING THIS SEAL. WE FOLLOWED THE AMM STEP-BY-STEP. AFTER REMOVING AND REPLACING THE SEAL; WE PERFORMED AN OPS TEST OF THE DOOR SEAL. WE DID NOT REALIZE THAT THE RESERVOIR WAS ALREADY FILLED TO CAPACITY DUE TO AVIONICS TESTS. THE AMM DID NOT INSTRUCT US TO DRAIN THE RESERVOIR. THE MANUAL STATES TO NOT GO ABOVE 19.5 PSI. WHEN SVCING BEGAN; A LARGE NOISE WAS HEARD FROM THE OTHER SIDE OF THE ACFT. THE AIRBORNE TEST KIT WAS INSTALLED AND PEOPLE WITHIN THE AREA AND MYSELF THOUGHT THAT A DEICE BOOT HAD RUPTURED. THE 'A' CHK CREW PERFORMED OPS CHKS AND NO DEFECTS WERE NOTED. WE CONTINUED WITH OUR TEST AND INFLATED THE DOOR SEAL 3 TIMES; ONCE WITH THE DOOR OPEN AND TWICE WITH THE DOOR CLOSED TO VERIFY A GOOD SEAL WITHIN THE ACFT. THE NON ROUTINE WAS NOT CLOSED OUT AT THE TIME OF TURNOVER. MISCOM BTWN AVIONICS AND MECHS COMBINED WITH A POSSIBLE LACK IN AMM REFS DID NOT LEAD ANYONE TO SUSPECT THE RESERVOIR. SUPPLEMENTAL INFO FROM ACN 780814: WE THEN HEARD A LOUD BOOM. AT THAT TIME I TURNED OFF THE NITROGEN TO THE RESERVOIR. WE THEN PERFORMED OPS CHKS OF THE DEICE SYS AND FIRE EXTINGUISHING SYS DUE TO THE FACT THAT THE SOUND DID NOT SEEM TO ORIGINATE FROM INSIDE THE ACFT; NO DISCREPANCIES WERE FOUND. MR X AND MS Y THEN TESTED THE DOOR SEAL WITH THE BLEED AIR KIT. THE DOOR SEAL INFLATED WHICH INDICATED THAT THE RESERVOIR WAS IN FACT INTACT. A NON-ROUTINE WAS WRITTEN TO FURTHER TROUBLESHOOT THE CAUSE OF THE LOUD BOOM. IT WAS THEN TURNED OVER TO NEXT SHIFT. UPON ARR AT WORK ON A LATER DATE; I WAS NOTIFIED THAT ON ACFT X THE AIR RESERVOIR IN QUESTION HAD RUPTURED AND THAT THE LINE LEADING TO THE DOOR SEAL HAD BEEN SHEARED IN SUCH A WAY THAT THE DOOR SEAL WAS STILL ABLE TO OPERATE INFLT. THIS EVENT OCCURRED DUE TO LACK OF COM BTWN MECHS; AND IMPROPER KNOWLEDGE OF THE AIR RESERVOIR SYS. SUPPLEMENTAL INFO FROM ACN 780602: WHILE ON LEAD ASSIGNMENT; I; AMONG OTHERS; HEARD A LOUD POPPING NOISE THAT SOUNDED LIKE IT CAME FROM THE R SIDE OF THE ACFT. MECHS WERE TESTING MAIN CABIN DOOR SEAL AND LE DEICE SYS. NO AIR WAS GOING TO ACFT AT TIME OF POP. THE MECHS RETESTED MAIN CABIN DOOR SEAL AND LEADING EDGE DEICE SYS MULTIPLE TIMES WITH NO DEFECTS NOTED. I WROTE A NON ROUTINE MAINT WRITE-UP STATING WHAT HAPPENED AND THAT A GENERAL VISUAL INSPECTION OF #2 NACELLE PNEUMATIC LINES AND VALVES WERE LOOKED AT AND DISCREPANCY COULD NOT BE FOUND AND ACFT WAS TURNED OVER TO NEXT SHIFT. UPON ARRIVING FOR DUTY ON A LATER DATE; I LEARNED THAT MCD RESERVOIR HAD COME APART AT THE END SEAM AND CAUSING OTHER DAMAGE TO RESERVOIR MOUNTING.