As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 1263, BOS-AUS DIVERTED TO JFK, CABIN PRESSURE STARTED CLIMBING AT 26,000 FT AND WOULD NOT MAINTAIN ALTITUDE (MAX CABIN ALTITUDE 9600FT). ACCOMPLISHED REMOVAL AND REPLACEMENT OF RIGHT PACK OUTLET FLEX BELLOW PER AMM 21-51-21. PERFORMED CPCS OPERATIONAL TEST AND ADJUSTMENT TEST PER AMM 21-31-00. GROUND TEST NORMAL. NO LEAKS NOTED.