What was found
AIR TURNBACK CABIN PRESSURE WITH SINGLE PACK OPERATION. AFTER LEVEL OFF, AT CRUISE ALTITUDE FL 250 CABIN BEGAN CLIMBING AT 500 FPM, SWITCHING RT PACK TO HIGH FLOW DID NOT REDUCE CABIN CLIMB. WHEN CABIN REACHED 10,000 DESCENT, TURNBACK TO PDX WAS INITIATED. PRESSURIZED A/C ON GROUND, FOUND AVIONICS FLOW CONTROL VALVE INOP. REPLACED AVIONICS FLOW CONTROL VALVE, WAITING ON (P/N & S/N FROM MAINTENANCE). CABIN PRESSURE CONTROLLER WAS REPLACED LP 7664566 AND AFT PRESSURE BULKHEAD BOOT SEAL WAS REPLACED ON LP 7664567 FINAL FIX. RECOMMENDATION: REMAIN OPEN PENDING REVIEW OF TEARDOWN REPORT.