What was found
CLIMBING OUT TO 14,000 FT ALTITUDE, ACFT CREW COULD NOT CONTROL CABIN PRESSURE IN MANUAL MODE ON THE PRESSURE CONTROL PANEL AND THE MASTER CAUTION WARNING LIGHT ILLUMINATED. A LOUD AIRFLOW NOISE WAS HEARD FROM BEHIND THE INSTRUMENT PANEL. THE CREW ELECTED TO RETURN TO THE AIRFIELD. UPON INVESTIGATION IT WAS DISCOVERED THAT THE MANUAL CONTROL KNOB FOR THE FWD OUTFLOW VALVE WAS NOT IN THE CLOSED POSITION. AN EVALUATION FLIGHT WAS REQUESTED AND WAS SUCCESSFUL. THE ACFT WAS THEN RELEASED FOR SERVICE.