What was found
DURING CRUISE FROM DTW TO SFO, UNABLE TO MAINTAIN CABIN PRESSURE IN AUTO, STANDBY, OR MANUAL. AFTER DESCENT TO FL140, ABLE TO USE MANUAL CONTROL. FLIGHT DIVERTED TO MSP AND LANDED WITHOUT INCIDENT. FOUND INSULATION STUCK IN LOWER PRESSURE RELIEF VALVE AND UPPER VALVE DOOR STUCK PARTIALLY OPEN. REPLACED BOTH UPPER AND LOWER CABIN PRESSURE RELIEF VALVE. OPERATIONAL CHECK OK.