FLC OF AN MD80 MADE EMER DSCNT DUE TO LOSS OF ACFT PRESSURIZATION AND RETURNED TO LAND.

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DAYLIGHT; VFR; BEAUTIFUL DAY. CLBING OUT OF ORLANDO; LEAVING APPROX FL260; I NOTICED IN MY EARS THAT CABIN SEEMED TO BE CLBING FAST. A LOOK AT THE CABIN ALT/VSI GAUGE SHOWED CABIN OVER 7000 FT AND CLBING AT LEAST 1500 FPM; AND THE 'FLOW' LIGHT ILLUMINATED. ALL OTHER INDICATIONS NORMAL. CAPT (WHO WAS THE MONITORING PLT FOR THIS LEG) MANUALLY CLOSED THE OUTFLOW VALVE; WHICH DID NOT AFFECT CABIN RATE OF CLB. WE DECIDED TO DSND AND WORK ON PROB. ZJX PROMPTLY CLRED US TO FL240. BY NOW WE REALIZED THAT THE CABIN WAS OVER 9000 FT AND CLBING RAPIDLY. WE DONNED OXYGEN MASKS; DISCONNECTED THE AUTOPLT; EXTENDED THE SPD BRAKES AND BEGAN WHAT AMOUNTED TO AN EMER DSCNT. WE NOTIFIED ZJX THAT WE REQUESTED 10000 FT AND WERE IMMEDIATELY CLRED TO 10000 FT. 'CABIN ALT' WARNING LIGHT AND HORN AND MASTER WARNING LIGHT CAME ON AS CABIN WENT THROUGH 10000 FT. CABIN REACHED 13000 FT DURING THE DSCNT (MASKS DID NOT DROP). LEVELED AT 10000 FT AND RETURNED TO ORLANDO. CABIN BEGAN TO PRESSURIZE; BUT WE STILL HAD SOME PROBS CTLING IT. MAINT FOUND A BLOWN DUCT ON R PACK WHICH WAS DUMPING ALL OF ITS SUPPLY INTO TAIL COMPARTMENT. ALSO FOUND ANOTHER LEAK IN AIR CONDITIONING SUPPLY LINE. NEVER THOUGHT I WOULD EVER DO AN EMER DSCNT FOR REAL; BUT I DID.