A B727-200 AT LEVEL OFF AT FL240 STARTED AN EMER DSCNT DUE TO UNABLE TO MAINTAIN CABIN PRESSURE ON ONE OPERATIVE PACK CAUSED BY A LEAKING AFT CARGO DOOR SEAL.

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WE DEPARTED LAX TO PDX WITH A L PACK MEL'ED UNDER MEL XY-Y WHICH ALLOWS PRESSURIZED FLTS ON ONE PACK ONLY UP TO FL250. AS WE LEVELED OFF AT FL240 THE CABIN NEVER STABILIZED AND CLIMBED STEADILY AT 500 FPM. WE RAN THE STABILIZATION ITEMS AND ABNORMAL CHECKLIST AS WE DESCENDED TO FL220. THEN FL180. AS THE CABIN REACHED 10000 FT WE WENT ON OXYGEN AND EXECUTED AN EMER DSCNT COORDINATED WITH CTR AND ASSOCIATED CHECKLIST. AS THE CABIN DSCNDED BELOW 10000 FT WE COORDINATED AN INFLT RE-RELEASE BACK TO LAX WITH OUR DISPATCH. WITH THE HELP OF BFL APCH WE ORGANIZED A 3500 POUND FUEL DUMP TO MEET OUR LNDG WEIGHT REQUIREMENTS THEN RETURNED TO AND LANDED SAFELY AT LAX. WE COMPLIED WITH FAR'S; POLICIES AND PROCES AS OUTLINED BY OUR COMPANY GENERAL OPERATING MANUAL AND ACFT OPERATING MANUAL AND ALSO USED GOOD CRM SKILLS TO LAND SAFELY. I THINK THAT OLDER AIRPLANES LIKE THE B727 EQUIPPED WITH CARGO DOORS (ALWAYS LEAKING) SHOULD NEVER BE ALLOWED UNDER ANY MEL TO FLY ON ONE PACK ONLY ABOVE 10000 FT MSL IN THE INTEREST OF SAFETY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE DIRECT CAUSE OF THE PRESSURIZATION FAILURE WAS THE LEAKING AFT CARGO DOOR SEAL. THE RPTR SAID WITH ONE PACK DEFERRED AS INOP AND SINGLE PACK OP THE AIRPLANE WILL JUST HOLD FL250 MARGINALLY WITH GOOD DOOR SEALS. THE RPTR SAID THIS IS AN OLD AIRPLANE AND NEEDS EXTRA CARE AND ATTENTION BUT DOES NOT GET THE NECESSARY MAINT.