What was found
ON CLIMB OUT, ENROUTE, CABIN ALT WARNING HORN SOUNDED. CREW REPORTED OUTFLOW VALVE WAS FULLY CLOSED IN AUTO, STANDBY, AND MANUAL PRESS MODES BUT THEY WERE NEVER ABLE TO GET CABIN PRESS. EMER DESCENT WAS ACCOMPLISHED. FLIGHT LANDED WITHOUT FURTHER INCIDENT. NO EMER WAS DECLARED. MAIN FOUND APU BLEED AIR DUCT PRESS SEAL BOOT IN AFT KEEL BEAM DISLODGED. RESECURED CLAMP AND BOOT. FOUND CABIN PRESS OUTFLOW VALVE BLADE SEALS LEAKING. REMOVED AND REPLACED CABIN PRESS OUTFLOW VALVE. OPS CHECK GOOD. PRESS A/C TO 6.5 PSI. NO LEAKS NOTED. CLEARED MEL ON LEFT PACK BY REPLACING LEFT PACK WATER SEPARATOR 35 DEGREE SENSOR (PN 6275701). LEFT PACK OUTPUT READS 4 DEGREES CENTIGRADE IN FULL COLD. A/C OK FOR SERVICE.