AN ACR FLC IN AN MLG DEPARTS UNDER MEL PROVISOS AFTER AN EXTENSIVE GND DELAY. THEY DISCOVER DURING THEIR CLBOUT THAT THE ACFT WILL NOT PRESSURIZE AND THEY RETURN TO THE ARPT FOR AN OVERWT LNDG. THEN THEY FIND THAT THE ORIGINAL MEL RELEASE WAS INCORRECT.

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THE ACFT WE WERE TO PICK UP ARRIVED AT THE GATE WITH A WRITE-UP 'UNABLE TO HOLD PRESSURE AT LOW THROTTLE SETTINGS.' THE RADIO RACK FAN HAD PREVIOUSLY BEEN DEFERRED. MAINT DID NOT WANT THE PAX BOARDED IMMEDIATELY DUE TO SOME CHKS THAT THEY SAID THEY NEEDED TO MAKE. A SHORT TIME LATER; MAINT DECIDED TO DEFER THE L AIRCYCLE MACHINE DUE TO AN EXCESSIVE AIR LEAK; AS THEY CALLED IT; OUT OF A WITNESS HOLE. THEY CALLED MAINT CTL AND THE DISPATCHERS REGARDING THE MEL ITEMS. WE LATER OBTAINED A NEW RELEASE WITH THE FOLLOWING 2 ITEMS LISTED: 1) L PACK INOP AND 2) RADIO RACK COOLING RAN INOP. I LOOKED UP THE NEW DEFERRAL IN MY MEL AND VERIFIED WITH MAINT THAT ALL THE DEFERRAL ITEMS FOR THE PACK WERE ACCOMPLISHED. WE BOARDED THE FLT AND BLOCKED OUT VERY CLOSE TO SCHEDULE; ABOUT 5 MINS LATE. DURING INITIAL CLB AT ABOUT 2000 FT I INFORMED THE CAPT THAT IT DID NOT APPEAR THE PRESSURIZATION WAS HOLDING THE CABIN AT THE DESIRED RATE OF CLB SELECTED. THE CAPT ELECTED TO LEVEL OFF THE ACFT AT 10000 FT. THE BEST THE PRESSURIZATION WAS ABLE TO HOLD THE CABIN WAS 7500 FT; ABOUT 1.5 PSI DIFFERENTIAL. WE ELECTED TO RETURN TO THE ARPT. THE WIND WAS 220 DEGS; 20 KTS; A DIRECT XWIND TO RWY 30. WE REQUESTED RWY 24 FROM THE TWR AND THAT WAS GRANTED. WE ALSO LANDED OVERWT. THE LNDG WAS SMOOTH AND UNEVENTFUL. AFTER RETURNING TO THE GATE; THE FLT WAS CANCELED. LATER IN THE DAY; I PERSONALLY WENT BACK TO THE MEL AND FOUND OUT THAT WITH ALL THE ATTN GIVEN TO THE PACK PROB AND INSURING A TIMELY DEP; EVERYONE INVOLVED IN THIS OP MISSED A VERY CRITICAL ITEM. IF YOU LOOK UNDER THE DEFERRAL OF THE RADIO RACK FAN; YOU FIND THAT ITS DEFERRAL REQUIRES BOTH PACKS TO BE FUNCTIONING. THIS REQUIREMENT WAS OVERLOOKED BY 7 DIFFERENT INDIVIDUALS -- THE MECH; LEAD MECH; SYS SPECIALIST; MAINT CTL; DISPATCH -- SINCE IT ENDED UP ON THE RELEASE; AND BOTH PLTS. I BELIEVE THE CAUSE OF THE PROB IS TWOFOLD. THE PRIMARY REASON I BELIEVE IS THE PUSH TO GET THE FLT OUT ON TIME; THEREBY NOT ALLOWING INDIVIDUALS TO TAKE THE TIME TO INSURE ALL THE REQUIREMENTS ARE MET. SECONDLY; HAVING HAD SO MANY PACK DEFERRALS IN ALL OUR BACKGNDS; ONE CAN BECOME COMPLACENT WITH THE PROCS AND NOT TAKE THE TIME TO REVIEW THE MEL HOW THE RELATED DEFERRALS WILL AFFECT THE SYS. SOLUTIONS ARE RATHER SIMPLE IN HINDSIGHT. JUST SLOW DOWN AND TAKE THE REQUIRED TIME TO INSURE ALL THE REQUIREMENTS ARE MET. THIS IS REALLY APPLICABLE TO ME SINCE I HAD THE BOOK OPEN PRIOR TO DEP AND ACTUALLY DISCUSSED THE LATEST DEFERRAL WITH MAINT; THE RADIO RACK MEL ITEM WAS JUST 2 PAGES FURTHER IN THE MANUAL.