As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF USING SYSTEM NR1 NOTICED AFT OUTFLOW VALVE STAYED FULL OPEN. OPPOSITE SYSTEM SELECTED, NO HELP, UNABLE TO CLOSE MANUALLY. WITH BOTH PACKS ON COULD MAINTAIN 8500 FT CABIN ALTITUDE AT 14000 FT. DUMPED FUEL TO SAFE LANDING WEIGHT (APPROX 2600 GALS). RETURNED TO AIRFIELD, LANDING NORMAL. REPLACED SYSTEM NR 1 AFT PRESS AND SAFETY VALVE (OUTFLOW VALVE) IAW 21-31-13. SYSTEM FUNCTIONAL CHECK SATISFACTORY IAW 21-31-00 PG 501. AIRCRAFT RETURNED TO SERVICE.