ACFT EQUIP PROB MALFUNCTION MANDATES AN EMER DSCNT AFTER LOSS OF CABIN PRESSURE.

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WE HAD BEEN LEVEL AT FL260 FOR ABOUT 10 MINS WHEN WE NOTICED A 50 FPM RATE OF CLB ON THE CABIN VSI. WE ATTEMPTED TO CORRECT THIS BY MOVING THE PACKS UP TO THE NEXT HIGHEST SETTING. THIS GAVE US A RATE OF DSCNT ON THE CABIN ALT. ONE OF THE PACKS; HOWEVER; OVERHEATED. WE ASKED ZAU FOR FL240 TO MEET AN ACFT LIMITATION WITH 1 PACK INOP. DURING THE DELAY BTWN THE REQUEST AND THE CLRNC TO FL240; WE SWITCHED TO MANUAL PRESSURIZATION. WE SUSPECTED AN OUTFLOW VALVE AS THE CAUSE OF THE CABIN'S INABILITY TO HOLD ALT. WHEN WE RECEIVED OUR CLRNC TO FL240; I BEGAN THE DSCNT. THE CABIN 'BLEW.' I NOTICED THE CABIN VSI PEGGED AT 2000 FPM AND THE PAX OXYGEN LIGHT ON; INDICATING THE MASKS HAD DROPPED. I PERFORMED THE 'RAPID DEPRESSURIZATION' MEMORY ITEMS. WE TOLD ZAU WE NEEDED LOWER; NOW! THEY STEP DSNDED US TO 10000 FT. WE PERFORMED AN EMER DSCNT. AFTER LEVELING AT 10000 FT; I REQUESTED A CLRNC TO FWA. THE ACFT WAS STABLE AND THERE WAS NO EVIDENCE OF STRUCTURAL DAMAGE. I DECIDED TO RETURN TO CINCINNATI. AFTER LNDG; MAINT DISCOVERED A 3 INCH SECTION OF THE CARGO DOOR SEAL HAD BLOWN OUT. MY CREW PERFORMED A TEXTBOOK EXAMPLE OF HOW THIS SIT SHOULD BE HANDLED. THE PAX; WHO HAD BEEN TOLD THEY WOULD NOT GET ANY PEANUTS IF THEY DIDN'T LISTEN TO THE FLT ATTENDANT'S PREDEP SAFETY BRIEFING; PERFORMED AS THEY WERE SUPPOSED TO.