What was found
WHILE CLIMBING THROUGH FL 270, A MUFFLED THUMP WAS HEARD FROM THE AFT CABIN. RESULTED IN LOSS OF CABIN PRESS WITH A CABIN CLIMB RATE OF APPROX 2700 FEET PER MINUTE. CABIN MASKS WERE MANUALLY DEPLOYED. UPON INSPECTION ON THE GND, A SIGNIFICANT BLEED AIR LEAK WAS NOTICED IN THE FWD SECTION OF THE REAR BAY WITH APU RUNNING. INSPECTED REAR BAY CABIN AIR DUCTING & FOUND BAND CLAMP COMPLETELY BACKED OFF OF CABIN AIR SUPPLY DUCT ON OUTLET END OF WATER SEPARATOR, SECURED BAND CLAMP AS REQUIRED & PRESSURIZED ACFT TO MAX DIFFERENTIAL PRESS USING APU NO LEAKS NOTED, PRESSURIZATION SYSTEM OPS CHECK GOOD. RE-PACKED CABIN O2 MASKS AS REQUIRED. ALL DONE IAW MM 21-30-00-501 & 35-20-01.