A B757-200 IN CRUISE AT FL350 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURE CAUSED BY A FAILED OUTFLOW VALVE.

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DEPARTED DFW WITH L PACK INOP. ON TKOF; L ENG BLEED OFF LIGHT APPEARED WITH ASSOCIATED EICAS. MEL SAID THIS MIGHT OCCUR AND SHOULD GO AWAY WHEN PWR IS REDUCED. THIS DID NOT HAPPEN; BUT NORMAL DUCT PRESSURE WAS PRESENT ON THE GAUGE. ACCOMPLISHED ABNORMAL. WHEN L BLEED AIR SWITCH WAS PRESSED OFF; DUCT PRESSURE WENT TO ZERO. WHEN TURNED BACK ON; LIGHT REMAINED ON; BUT DUCT PRESSURE RETURNED TO NORMAL. CONTINUED ON WITH FLT; WITH ALL OTHER INDICATIONS NORMAL -- CABIN WAS IN NORMAL SCHEDULE. AT FL350; FO WENT BACK TO CHK OUT A PROB WITH THE FIRST CLASS LAVATORY. RIDE STARTED TO BECOME MODERATE CHOP. I ASKED ATC FOR FL280; WHICH WAS RPTED SMOOTH. I USED FLT LEVEL CHANGE; BRINGING PWR TO IDLE TO DO A QUICK DSCNT THROUGH CHOP. DUCT PRESSURE FOR BOTH ENGS STAYED AROUND 30-35 PSI. AROUND FL310; CABIN ALT WARNING HORN AND ASSOCIATED MESSAGES AND LIGHTS APPEARED. I WAS ON OXYGEN AT THE TIME; WITH FO GONE; STARTED EMER DSCNT AND CALLED FOR THE FO OVER THE PA. AN EMER WAS DECLARED. WHEN WARNING STARTED; CABIN ALT WAS SLIGHTLY OVER 10000 FT. FO CAME UP AS SOON AS I CALLED FOR HIM. HE IMMEDIATELY WENT ON OXYGEN; ASSESSED THE PROB AND STARTED RIGHT IN. THE CREW COORD KICKED RIGHT IN. HE FLEW THE AIRPLANE WHEN NEEDED; HANDLED THE PRESSURIZATION; KEEPING THE CABIN TO NO HIGHER THAN 11500 FT BY GOING TO MANUAL. THE FO BROUGHT THE CABIN BACK DOWN TO NORMAL SCHEDULE; WHILE I DSNDED TO 10000 FT. FLT ATTENDANTS WERE NOTIFIED OF THE PROB; THAT WE WERE RETURNING TO DFW; NO EMER EVAC WAS NEEDED. DURING THE DSCNT; WE SAY THE STATUS MESSAGES OF CABIN AUTO #1; CABIN AUTO #2. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF CABIN PRESSURE WAS CAUSED BY A FAILED OUTFLOW VALVE. MAINT REPLACED THE OUTFLOW VALVE AND PRESSURIZATION SYS #1 AND #2 OPERATED NORMALLY.