What was found
AIRCRAFT RETURNED FLIGHT BECAUSE IN AUTOMATIC MODE THE PRESSURIZATION SYSTEM HAD OSCILLATIONS AND IN MANUAL MODE THE PRESSURE VALVE COULD NOT BE CLOSED. DURING MAINTENANCE REVISION, THE SWITCH WAS PLACED IN MANUAL POSITION AND VALVE WAS CLOSED, THE SYSTEM OPERATED CORRECTLY. DUE TO OXYGEN MASKS AND OXYGEN GENERATORS WERE DEPLOYED AIRCRAFT WAS SENT TO THE HANGAR AND LEAK TEST WAS PERFORMED IAW MM 21-00-00, PRESSURE RELIEF VALVES WERE CHECKED OPENING AT 8 PSI, AND OPERATING CORRECTLY. OXYGEN MASKS AND OXYGEN GENERATORS WERE REPLACED, ALL TESTS WERE CORRECT.