What was found
AT FL380 EICAS DISPLAYED AMBER CABIN ALT AT 8400 AND RATE OF 100 FT/MIN. CABIN CONTINUED TO CLIMB. TRIED TO CONTROL MANUALLY BUT OUTFLOW VALVES WERE ALREADY FULL CLOSED. REQUESTED DESCENT TO 360 BUT CABIN CONTINUED TO CLIMB. EVENTUALLY CABIN WENT OVER 10000 FT. WE BEGAN A DESCENT TO FL250 INITIALLY BUT CABIN STAYED ABOVE 10000 FT. WE EVENTUALLY WENT TO 9000 FT. L & R PACK FCV STATUS MESSAGE DISPLAYED DURING DESCENT. CABIN CAME BELOW 10000 FT AT ABOUT 17000 FT. AIR TURN BACK TO PANC. REPLACED UPPER POSITIVE PRESSURE RELIEF VALVE PER M.M. 21-32-01-2, LEAK CHECKED STATIC LINE PER SUBTASK 21-32-01-210-004, TESTS PASSED, OK FOR SERVICE.