What was found
AIRCRAFT WAS GROUNDED. ON CLIMB OUT, WE HAD A MASTER WARNING AND CABIN PRESSURE EXCESS ALTITUDE WITH CABIN PRESS OF 9.8 AND CLIMBING. MADE EMERGENCY DESCENT TO 10000 AND DIVERTED TO STL ACCOUNT OF PRESSURIZATION PROBLEM. MASKS DID NOT DROP AND CABIN PRESSURE CAME DOWN WITH DESCENT. EMERGENCY DECLARED AND LANDED WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. MAINTENANCE REPLACED GASKET ON COMPRESSOR INLET PER AMM 21-52-21 AND ACCOMPLISHED SATISFACTORY OPERATIONAL CHECK.