What was found
(CAN) DURING CLIMBOUT, SEPT 16 2008, PRESSURIZATION FAILED TO OPERATE NORMALLY. WHEN CLIMBING THROUGH 23,000 FT, CABIN ALTITUDE EXCEEDED 10000 FT. FUEL DUMP CARRIED OUT IN ORDER FOR ACFT TO RETURN TO DEPARTURE. CABIN PRESSURIZATION LEAK CHECK CARRIED OUT, NO FAULTS. PRESSURIZATION CONTROLLER AND PRESSURIZATION CONTROL PANEL REPLACED. GROUND PRESSURIZATION CHECKS CARRIED OUT WITH NO FAULTS DETECTED. ON SEPT 17, 2008, ACFT WAS DEPARTED. AS THE ACFT CLIMBED THROUGH 25000 FT, CABIN PRESSURIZATION FAILED TO OPERATE NORMALLY WITH MINIMAL CONTROL. ACFT RETURNED TO DEPARTURE FOR FURTHER TROUBLESHOOTING. TROUBLESHOOTING REVEALED NO PROBLEMS WITH THE PRESSURIZATION SYS BUT HAS REVEALED THAT THE RT PACK FLOW CONTROL AND SHUTOFF VALVE FAILED TO OPERATE NORMALLY. THIS RESULTED WITH A LOSS OF 50 PERCENT OF PRESSURIZATION AIR. ACFT IS CURRENTLY OUT OF SERVICE AWAITING ARRIVAL OF A SERVICEABLE PACK FLOW CONTROL SHUTOFF VALVE.