What was found
AT FL 270 CABIN ALTITUDE AURAL WARNING SOUNDED AND ALTITUDE ALERTER ILLUMINATED AND AURAL WARNING SOUNDED. UNABLE TO CONTROL CABIN ALTITUDE EXECUTED EMERGENCY DESCENT RETURNED TO DEPARTURE AIRPORT. (AUS) T/S SYS, RAN ENGINES AND PERF DECAY CK PER M/M 05-51-71, CKD WITHIN LIMITS. SUSPECTED EQUIPMENT COOLING BLOWER CTRL VALVE OF BINDING. REMOVED AND REPLACED VALVE PER M/M 21-58-21. SUSPECT LACK OF AIR FLOW FROM LT PACK. REMOVED AND REPLACED LEFT PACK VALVE PER M/M 21-55-71. PERF OPS CK OF PRESS CTRL SYS IAW.