What was found
(CAN)AIRCRAFT RETURNED TWICE DUE TO PRESSURIZATION PROBLEMS, OUTFLOW AND CONTROLLER REPLACED. NEXT FLIGHT PRESSURIZATION FAILED, REPLACED PRESSURE CONTROL PANEL AND COOLING EQUIPMENT FAN EXHAUST VALVE. NEX T FLIGHT UNABLE TO CONTROL PRESSURIZATION AT FL310 AND 7.6 PSI DIFFERENTIAL. REPLACED UPPER AND LOWER PRESSURE RELIEF VALVES. TEST FLOWN TO FL370 OK.