What was found
SFO - FL78 - DURING CLIMB-OUT FROM SFO, THE CABIN ALTITUDE ALERT HORN SOUNDED WHEN CABIN PRESSURE CLIMB TO 13,000 FEET AS AIRCRAFT APPROACHED FL 330. DESCENT WAS INITIATED AND CABIN PRESSURE WAS REGAINED APPROACHING FL250. LEFT PACK PRESSURE INDICATED ZERO PSI. FLIGHT RETURNED TO FIELD AT SFO AND LANDED WITHOUT FURTHER INCIDENT. NO OXYGEN MASKS WERE DEPLOYED. MAINTENANCE REMOVED AND REPLACED THE NR 1 ENGINE 450 DEGREE BLEED THERMOSTAT. OPERATIONAL CHECK PER MM 36-11-05 GOOD. (M)