As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAKE OFF, THE FLIGHT CREW FOUND THE CABIN PRESSURIZATION COULD NOT BE MAINTAINED. MAINTENANCE FOUND THE CHECK VALVE SEAL ON THE OUTFLOW VALVE HAD SEPARATED FROM THE VALVE ASSEMBLY DUE TO A BROKEN RIVET AND SLIPPED FLAPPER.MAINTENANCE REMOVED AND REPLACED THE OUTFLOW VALVE IN ACCORDANCE WITH THE REQUIREMENTS OF BOEING MAINTENANCE MANUAL 21-31-00, PAGE 201.