What was found
30 MINUTES INTO FLIGHT AT FL340 EXPERIENCED "CABIN ALTITUDE" ALERT. NOTED A DIFFERENTIAL OF 6.0 WITH CABIN ALT OF 11,000. RAN QRH AND PROCEEDED TO KICT AT 10.000 FT.. MODE SELECTOR WAS IN AUTO 1 WHEN THIS OCCURED. WHILE TROUBLE SHOOTING NOW FOUND THAT THE EQUIPMENT COOLING OVERBOARD EXHAUST VALVE WILL NOT CLOSE AT ALL DURING THE PRRESURIZATION LEAK CHECK WHICH KEPT THE AIRCRAFT FROM PRESSURISING. REMOVED AND REPLACED THE MARKED BELLY DRAINS AND THE EQUIPMENT COOLING OVERBOARD EXHAUST VALVE REF B757 AMM 21-58-05-4 OPS CHECK GOOD. PERFORMED LEAK CHECK REF B757 AMM 05-51-24-2. LEAK CHECK GOOD. A/C OK FOR SERVICE.