As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
Aircraft at F/L 280 outflow valves full closed cabin climbing at 2,000 ft/min. Manual and stby control no help. Aircraft returned to departing airport. Fuel dumped (42K lbs.) to make max landing wt. Aircraft landed normal.Corrective Action: Mx found duct out of position at ATM compartment. Repositioned and secured duct. Pressurized A/C. System check satisfactory on ground I/A/W 36-14-02 and 21-31-00 of MM. Aircraft returned to service.