What was found
BOI- CLIMBING THROUGH 25,000 FEET CABIN PRESSURIZATION WAS CLIMBING 8,000 FEET. LEVELED OFF AT 26,000 FEET, CABIN PRESSURE CONTINUED TO CLIMB TO 10,000. (AIRFOIL ANTI-ICE WAS ON AND WING TEMPERATURE WAS IN YELLOWAND BACK-UP VALVE WAS CLOSED).AIRCRAFT RETURNED TO FEILD WITH OUT INCIDENT.ON DECENT TO BOI TURNED AIRFOIL OFF, BACK-UP VALVE OPENED AND PRESSURE RETURNED TO 55 PSI.REMOVED AND REPLACED #1 BACK-UP VALVE AND #2 MAIN VALVE. OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.