What was found
AFTER TAKEOFF AT STT - CREW REALIZED CABIN COULD NOT BE PRESSURIZED. BLEADS OFF AND RE-ESTABLISHING PRESSURIZATION, WE NOTICED THE OUTFLOW VALVE WAS NOT CLOSED AND WOULD NOT CLOSE MANUALLY. THEY LEVELED OFF AT 5,000 FEET, DECLARED AN EMERGENCY AND LANDED WITHOUT INCIDENT. THE AIRCRAFT WAS GROUNDED. MAINTENANCE CHECKED RESET PRESSURIZATION CONTROLLER C/B F3, F5, F6, F7. ACCOMPLISHED OUTFLOW VALVE OPERATIONAL TEST, AUTO SYSTEM, ALTERNATE SYSTEM AND MANUAL SYSTEM VALVE - OPERATIONS NORMAL, OPEN AND CLOSE NORMAL, NO FAULTS DURING TEST PER AMM 21-31-00.