What was found
CLT - FLT923 EN ROUTE FROM CLT TO ORD, CLIMBING THROUGH FL250, THE CABIN ALTITUDE EXCEEDED 10,000 FEET AND WAS CLIMBING 1,500 FEET PER MINUTE WITH OUTFLOW VALVE SHOWING FULL CLOSED. FLIGHT DESCENDED TO 10,000 FEET AND RETURNED TO FIELD AT CLT. FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND THE LEFT-HAND SAFETY RELIEF VALVE LEAKING. REPLACED VALVE PER MM 21-31-00. PRESSURIZED AIRCRAFT. OPERATIONAL CHECK OF SYSTEM GOOD.