What was found
AIRCRAFT 860 DEPARTED LAS FLT 418 AND CLIMBING THROUGH 18,000 FEET THE CABIN PRESSURIZATION FLOW LIGHT ILLUMINATED. CREW WAS UNABLE TO EXTINGUISH LIGHT AND RETURNED TO LAS. MAINTENANCE DEPARTMENT PERSONNEL INSPECTED THE PRESSURIZATION SYSTEM AND DISCOVERED INSULATION IN THE OUTFLOW VALVE LINKAGE. REMOVING THE DEBRIS THE PRESSURIZATION SYSTEM OPERATIONAL CHECK PERFORMED IN A SATISFACTORY CONDITION. THE AIRCRAFT WAS RELEASED AND RETURNED TO SERVICE.