What was found
ON CLIMBOUT, CREW REPORTED PRESSURIZATION SYSTEM WILL NOT MAINTAIN CABIN ALTITUDE. AIRCRAFT RETURNED BACK TO DEPARTURE AND LANDED WITHOUT ANY FURTHER INCIDENT. MAINTENANCE TROUBLESHOT PRESSURIZATION SYSTEM AND FOUND FLOW CONTROL AND MODULATING VALVE AT FAULT, THE FLOW CONTROL AND MODULATING VALVE WAS REMOVED AND REPLACED IAW MM CH-36-12-61, PRESSURIZATION SYSTEM WAS OPERATIONALLY CHECKED AND FOUND NORMAL IAW W/C 7-29 AND MM CH-5-51-101. AIRCRAFT WAS RETURNED TO SERVICE.