What was found
DURING CLIMB AT 9,000 FT, CREW REPORTED ACFT UNABLE TO KEEP PRESSURIZATION AND CABIN PRESSURE CLIMBING AT SAME RATE OF ACFT. CREW STARTED DESCENT JUST AS CABIN ALTITUDE AMBER MESSAGE NOTED. CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND FOUND SECURITY SEAL ON AFT CARGO DOOR. MAINTENANCE REMOVED SECURITY SEAL AND PERFORMED CABIN PRESSURIZATION IAW AMM. NO DEFECTS NOTED. ACFT RELEASED FOR SERVICE.