As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE TROUBLESHOOTING A PRESSURIZATION PROBLEM, A CRACKED CHECK VALVE AT THE RT PRESSURE REGULATING AND CHECK VALVE ASSEMBLY WAS FOUND. A PIECE OF THE CHECK VALVE WAS MISSING. THE LT SIDE WAS THEN CHECKED AND AN IDENTICAL CHECK VALVE CONDITION WAS FOUND. A SEARCH FOR THE MISSING PIECE WAS UNSUCCESSFUL.