What was found
NO PRESSURIZATION FAULTS WERE NOTED DURING PREFLIGHT CHECKS. DURING CLIMBOUT THE CABIN ALTITUDE INCREASED WITH AMBIENT ALTITUDE. PRESSURIZATION CHECKLISTS WERE FOLLOWED AND THE ACFT RETURNED TO DEPARTURE AIRPORT AND LANDED WITHOUT INCIDENT. GROUND OPS CHECKS FOUND LOW CABIN AIR INFLOW WHEN "CABIN AIR" SWITCH WAS TURNED ON. TROUBLESHOOTING REVEALED THAT 2 OF THE 4 LT BLEED AIR CHECK VALVE SEGMENTS WERE MISSING. CHECK VALVE SEGMENTS WERE FOUND LODGED IN THE FLOW CONTROL VALVE CAUSING IT TO HANG UP AND NOT OPERATE CORRECTLY. SEGMENTS WERE REMOVED AND THE ACFT WAS REASSEMBLED, ALL FUNCTIONAL CHECKS WERE SATISFACTORY.