As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL340 CABIN ALT EICAS MESSAGE AND DISCRETE LIGHT ILLUMINATED AND STAYED ON UNTIL DESCENT TO 13,000. CREW DECLARED AN EMERGENCY AND OXYGEN MASKS WERE USED. AIRCRAFT RETURNED TO DEPARTURE AIRPORT WITHOUT FURTHER INCIDENT. MAINTENANCE GROUNDED AIRCRAFT. MAINTENANCE FOUND LEFT BLEED OUTPUT VERY LOW. REMOVED AND REPLACED THE LEFT, PRV IAW AMM 36-11-10 PRV CONT. IAW AMM 36-11-19 AND THE LEFT PRSOV IAW AMM 36-10-09 ALL 3 COMPONENTS OPS NORMAL AIRCRAFT REMAINS ETOPS 180. AIRCRAFT RETURNED TO SERVICE