What was found
PRIOR TO PUSHBACK, FLIGHT CREW EXPERIENCED AN APU FIRE WARNING, RED LIGHT AT HANDLE & EICAS MSG & AURAL WARNING. SUBSEQUENTLY FOLLOWED BY AN APU FIRE EXTINGUISHER AUTO-DISCHARGE, BOTTLE DISCHARGE LIGHT & APU SHUTDOWN. 20 MINUTES AFTER INITIAL ALARM THE WARNINGS WERE AGAIN ANNUNCIATED. AT THIS TIME THE FIRST OFFICER PULLED THE APU FIRE HANDLE & TWISTED IT LEFT TO DISCHARGE BOTTLE AS PRECAUTION, NO EVIDENCE OF FIRE. WHILE ACCOMPLISHING APU INSPECTION, FOUND THE COMBUSTION CHAMBER DRAIN VLV GASKET BLOWN OUT. DRAIN VLV LOOSE. THIS WOULD CAUSE VERY HOT AIR TO BLOW ONTO FIRE LOOPS WHICH ARE LOCATED IN CLOSE PROXIMITY.