What was found
AT FL330, CREW REPORTED CABIN BEGAN A 6,000 FPM CLIMB. AUTO FAIL LIGHT ILLUMINATED ALONG WITH STANDBY ON LIGHT. CREW SWITCHED TO MANUAL AC/DC PRESSURIZATION MODE AND CONFIRMED OUTFLOW VALVE FULLY CLOSED. CREW APPLIED THRUST ON ENGINES AND DESCENDED TO LOWER ALTITUDE TO CONTROL CABIN. FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED BOTH CABIN PRESSURE SAFETY RELIEF VALVES PER MM 21-32-21. OPERATIONAL CHECK AND LEAK CHECK GOOD. PERFORMED SATISFACTORY TEST FLIGHT. AIRCRAFT OK FOR SERVICE.