As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT PACK WILL NOT HOLD CABIN ALTITUDE BELOW 10000'. AT FL 340, CABIN ALTITUDE ALARM SOUNDED. CABIN RETURNED TO AN ALTITUDE BELOW 10000' AT FLT 280. CREW ELECTED TO DO AN AIR TURNBACK. AIRCRAFT LANDED WITHOUT FURTHER INCIDENT. AIRCRAFT WAS GROUNDED BY MAINTENANCE. MAINTENANCE FOUND NUMEROUS PNEMATIC LEAKS ON DUCTING AND REPAIRED THE LEAKS. REPLACED ISOLATION VALVE IAW B767 AM 21-23-07. OPS CHECK GOOD. AIRCRAFT RETURNED TO SERVICE