What was found
CREW REPORTED THE ACFT WOULD NOT HOLD PRESSURIZATION, 8,800 CABIN & CLIMBING AT 12,000. VENT AVIONICS SYS FAULT. FLIGHT RETURNED TO FIELD & LANDED WITHOUT INCIDENT. FOUND SKIN AIR EXTRACT VLV, MEL 21-26-04A MAIN FLAP OPEN. COMPLIED WITH MEL 21-26-04A, CLOSED MAIN FLAP & OPENED INTERNAL FLAP. OPERATIONS TEST CHECKED IAW TSM 21-26-00-810-829-A.