What was found
IN CRUISE FLIGHT AT FL270 AN AIR LEAK AT THE AFT PRESSURE BULKHEAD DOOR WAS REPORTED. THE RED "CABIN PRESS" WARNING LIGHT ILLUMINATED AS THE CABIN ALTITUDE REACHED 10,500 FEET. THE MASKS DID NOT DEPLOY. THE RED BORDER CHECKLIST AND DESCENT TO 11,000 FEET WERE ACCOMPLISHED. CABIN PRESSURE WAS STABILIZED DURING DESCENT. LANDING AT MSP WAS UNEVENTFUL AND MAINTENANCE FOUND THE AFT CABIN PLUG DOOR IMPROPERLY SEATED. AFTER INSPECTION OF THE DOOR SEAL AND FRAME THE CABIN PRESSURE WAS CHECKED. NO LEAKS WERE FOUND SO THE AIRCRAFT WAS RETURNED TO SERVICE.