B737 DISPATCHED WITH FORWARD OUTFLOW VALVE INOP MEL'ED AND L AIR CONDITIONING COMPRESSOR INOP MEL'ED. CREW ASKED DISPATCH FOR MAX ALT ON NEW EQUIP RELEASE AND IT WAS SHOWN AS 25000 FT INSTEAD OF THE LEGAL 20000 FT. AT FL220 CABIN WAS LOSING PRESSURE. EMER DECLARED AND RETURNED TO MCI.

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ORIGINAL DISPATCH HAD 2 MEL'S (TRAY TABLE IN ROW 22 INOP AND FORWARD OUTFLOW VALVE FAILED OPEN). DURING PREFLT WALKAROUND INSPECTION; I NOTICED THE L PAC (PRESSURIZATION CTLR) ON L SIDE OF ACFT BAY OPEN AND MAINT TECHNICIAN WORKING ON PAC. HE SAID PAC HAD BLOWN DUCT AND WOULD BE MEL'ED. I NOTIFIED THE CAPT UPON MY RETURN TO THE COCKPIT AND LOOKED UP INOP PAC IN MEL BOOK. THE CAPT CALLED DISPATCH FOR AN AMENDED RELEASE AND ASKED THEM TO DOUBLECHK THE MAX ALLOWABLE ALT WITH CURRENT MEL'S. 5 MINS PRIOR TO PUSH; AN AMENDED RELEASE WAS BROUGHT TO THE COCKPIT WITH FL250 LISTED AS MAX ALT WITH 1 PAC INOP. IT WAS MY LEG TO FLY; AND TAXI OUT/TKOF WERE UNEVENTFUL. PASSING FL220 ENRTE TO FL240; I DID A QUICK CHK OF THE OVERHEAD PANEL AND NOTICED CABIN ALT WAS APCHING 10000 FT AND INCREASING AT 1000 FPM. I BROUGHT THIS TO THE CAPT'S ATTN AND ASKED HIM TO GET A LOWER ALT. CABIN ALT INCREASED TO A MAX OF 10500 FT BEFORE REVERSING. RETURN TO MCI AND LNDG WAS UNEVENTFUL. SUPPLEMENTAL INFO FROM ACN 365380: MAX ALLOWABLE ALT ON DISPATCH RELEASE SHOULD HAVE STATED FL200; BUT ERRONEOUSLY STATED FL250. WAS UNABLE TO MAINTAIN CABIN ALT BELOW 10000 FT PRESSURE ALT. DECLARED AN EMER WITH ATC AND RETURNED TO MCI.