What was found
FLIGHT RETURNED, UPON CLIMB OUT, THE FLIGHT CREW WAS UNABLE TO PRESSURIZE THE ACFT & RETURNED, WHERE THE ACFT LANDED UNEVENTFULLY. NO EMER DECLARED. MAINTENANCE FOUND THAT THE SKIN VENT SYS HAD ENCOUNTERED A FAULT & REPLACED THE SKIN AIR OUTLET VLV & ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK IAW AMM 21-26-53-000-001-4 400.