What was found
AIRCRAFT WOULD ONLY PRESSURIZE TO .5 PSI DIFFERENTIAL 8,500 FEET CABIN ALT AT 10,000 AIRCRAFT ALT, NOTED 26 PSI SPLIT ON DUCT PRESSURE GAUGES. REMOVED AND REINSTALLED LEFT OUTFLOW VALVE. REMOVED AND REPLACED NR 1 ENG 13TH STAGE MOD VALVE AND LEFT PACK 35 DEG CONTROL VALVE. GROUND RAN ENG, OPS CHECK NORM.