EMER DSCNT FOLLOWING A GRADUAL LOSS OF PRESSURIZATION.

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WE RETURNED TO THE GATE AT SLC DUE TO A PROB WITH OUR R AIRCONDITIONER. THE PROB COULD NOT BE CORRECTED; SO WE WERE DISPATCHED WITH THE R AIRCONDITIONER INOP. THIS MEL CONDITION RESTR OUR MAX FLT ALT TO FL350. AFTER TKOF; WE CLBED TO AND LEVELED OFF AT FL330. DURING CLBOUT; CABIN PRESSURIZATION WAS CHKED OFTEN AND APPEARED NORMAL. AFTER SEVERAL MINS AT CRUISE ALT; WE RECEIVED THE 'CABIN ALT' WARNING. THE PRESSURIZATION CTL PANEL SHOWED A CABIN ALT OF 10000 FT WITH A CABIN RATE OF CLB OF 200-300 FPM. THE OUTFLOW VALVE SHOWED CLOSED. WE DONNED OUR OXYGEN MASKS AND REQUESTED AN IMMEDIATE DSCNT. AFTER DECLARING AN EMER WITH ATC; WE WERE CLRED TO FL240 (OR THEREABOUTS). THE PWR REDUCTION REQUIRED FOR DSCNT CAUSED THE CABIN TO CLB AT 1000 FPM. WE CONTINUED OUR DSCNT TO FL240. THE CABIN ALT WAS NOW AT APPROX 1200 FT AND CLBING. WE DECLARED AN EMER SEVERAL MORE TIMES REQUESTING A DSCNT TO MEA AND SQUAWKED 7700 WITH THE XPONDER. THE CTLR DIDN'T REALIZE THE TIME CRITICAL NATURE OF OUR DSCNT AND SAID HE WOULD GIVE US A LOWER ALT AFTER PASSING TFC. WE EMPHASIZED THAT WE WERE AN EMER ACFT AND NEEDED FURTHER DSCNT. FURTHER DSCNT WAS GRANTED AFTER ATC TURNED ANOTHER ACFT. WE DSNDED TO 16000 FT; AND WE RECEIVED A CLRNC TO SLC. CABIN PRESSURIZATION WAS RECOVERED AND CABIN ALT BEGAN TO DSND SOMEWHERE BTWN FL240 AND 16000 FT. PAX OXYGEN MASK DID NOT DEPLOY AS THE CABIN DID NOT GET ABOVE 14000 FT. ON THE RETURN TO SLC; WE RECEIVED A 'CABIN ALT AUTO 1' STATUS MESSAGE. SEVERAL MINS LATER WE RECEIVED A 'L PACK TEMP' EICAS MESSAGE AND A L PACK 'INOP' LIGHT ON THE AIR- CONDITIONING PANEL. AN UNEVENTFUL RETURN TO SLC ARPT WAS ACCOMPLISHED. ALL NECESSARY CHKLISTS WERE ACCOMPLISHED AT THE APPROPRIATE TIME. NO PAX OR CREW MEMBERS WERE HURT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE B-757 AND DOES NOT KNOW WHAT CAUSED THE PRESSURIZATION PROB. HE KNOWS THAT SEVERAL 'BLACK BOXES' WERE CHANGED. HE BELIEVES THAT HE TOOK OFF 'WITH THE BETTER PACK' DISABLED PER MEL. HIS ACR'S FAA LIAISON TALKED DIRECTLY WITH THE ARTCC CTLR THAT DENIED THE LOWER ALT. THE CTLR 'HAD A PLAN' FOR GIVING A LOWER ALT AFTER TFC PASSED. THE RPTR'S TRIP WAS CANCELED AND HE HAS NOT FLOWN THIS ACFT SINCE THE DAY OF THE INCIDENT.