As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
OUT OF SVMC, DURING CLIMB AT 26,000 FEET, THE CABIN STARTED TO CLIMB AT MAXIMUM RATE THE CABIN WAS UNCONTROLLABLE, FOLLOWED THE CHECKLIST AND RETURNED TO SVMC WITHOUT INCIDENT. AIRCRAFT WAS PRESSURIZED USING ENGINE 1 AND 3 BLEEDS. THE PROBLEM WAS THE NR 3 ENGINE 13TH STAGE MOD VALVE WAS INTERMITTENT. THE SAME WAS PLACED ON DMI PER MEL 36-11-1. AIRCRAFT WAS RETURNED TO SERVICE. (M)