What was found
DURING CRUISE MASTER WARNING ILLUMINATED AT FL400 AND CABIN ALTITUDE WARNING EICAS DISPLAYED WITH SIREN , RAN QRH TO COMPLETION . CABIN ALT MODE SELECTOR IN AUTO 2. CREW DECLARED EMERGENCY PERFORMED EMERGENCY DECENT TO 10,000 AND FINISHED FLIGHT INTO CVG. AFTER LANDING MAINTENANCE GROUND AC AND TROUBLESHOT FINDING AIR DRIVEN PUMP SHUT OFF VALVE PARTIALLY OPEN WHEN OFF,AUTO,OR ON. REPLACE VALVE IAW 29-11-30 OPS CHECK GOOD,ALSO PERFORMED OPS CHECKS ON LH AND RH PACKS BOTH OPERATING NORMALLY IAW B767 AMM 21-51-00 . AC RETURNED TO SERVICE .