What was found
(CAN) DURING FLIGHT AT 5,000 FEET ASL, FLIGHT CREW NOTICED A GRADUAL LOSS OF CABIN PRESSURIZATION UNTIL THE AIRCRAFT CABIN ALTITUDE MATCHED AIRCRAFT ALTITUDE. AIRCRAFT RETURNED TO BASE UNPRESSURIZED. AIRCRAFT PRESSURIZATION SYSTEM GROUND CHECKED WITHOUT FAULTS. DOOR SEAL LINE PURGED AS A PRECAUTION. AIRCRAFT TT: 2,277.