What was found
WHILE CRUISING AT FL370 BETWEEN THE CALIFORNIA AND NEVADA BORDER, THE FO AND CAPT HEARD A BEEPING NOISE COMING FROM THE MAIN DECK CARGO AREA. ABOUT 30 SECONDS AFTER THAT THEY RECEIVED A MASTER WARNING EICAS CABIN ALTITUDE. AT THAT TIME THEY BOTH DONNED OXYGEN MASKS AND ESTABLISHED CREW COMMUNICATIONS & DECLARED AN EMERGENCY WITH ATC (EDC). THEY THEN AIR DIVERTED TO KLAS (DIV). AFTER LANDING FIRE TRUCKS DID A VISUAL INSP ON THE EXTERIOR OF ACFT TO SEE IF THERE WAS ANY VISIBLE DAMAGE; WHICH THERE WASN’T & CONTINUED TO TAXI AND PARK. MTX GROUNDED AC , MTX TROUBLESHOT AND REMOVED AND REPLACED PRV CONTROLLER IAW AMM 36-11-19.AND RH ENGINE PRSOV PRECAUTIONARY IAW AMM 36-11-09 OPS CHECK GOOD. ACFT REMAINS NO ETOPS