A B757-200 FLT CREW WAS DISPATCHED WITH R PACK INOP; AND EXPERIENCED EICAS CABIN ALT ALERT IN CRUISE FLT. THEY TURNED THE R PACK ON TO REGAIN PRESSURIZATION CTL; AND RETURNED TO DEP STATION.

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FO DISCOVERED A SMALL AMOUNT OF WATER DRIPPING FORWARD OF LOWER BEACON ON WALKAROUND. MAINT FOUND SMALL LEAK IN R PACK COLD WATER INJECTOR(?) ALTHOUGH WE WERE DISPATCHED WITH R PACK INOP; MAINT SUGGESTED R PACK WAS OPERABLE. ENRTE AFTER LEVELOFF AT FL340; RED CABIN ALT EICAS AND HORN CAME ON. FO AND I DONNED OXYGEN MASKS; NOTICED SLIGHT CLB IN CABIN RATE AND CABIN ALT AT 10000 FT. I IMMEDIATELY TURNED ON R PACK WHICH REVERSED THE CABIN INTO A 500 FPM RATE OF DSCNT. ABOUT 3 MINS LATER WHEN CABIN WENT BELOW 8500 FT AND LIGHT WENT OUT; WE TOOK OFF OUR MASKS AND I CONTACTED MAINT THROUGH DISPATCH. MAINT ADVISED US HE THOUGHT OPERATING PACK ENRTE SHOULDN'T BE A PROB BUT IT WAS MY DECISION. MY INITIAL DECISION WAS TO CONTINUE. AFTER FURTHER DISCUSSION WITH MY FO WE DECIDED IT WAS MORE PRUDENT TO EITHER LAND IN ORD OR JFK. WE THEN ADVISED DISPATCH AND RETURNED TO JFK WITH THE R PACK ON ENRTE. THIS ACFT HAD MULTIPLE R PACK WRITE-UPS OVER THE PRIOR MONTH WHICH MAY OR MAY NOT HAVE CONTRIBUTED TO OUR EXPERIENCE.