FK10 RETURNED TO LAND AFTER THE CABIN FAILED TO PRESSURIZE. MEL PROCS THAT WERE IN PLACE FOR A FERRY FLT TO MAINT HAD NOT BEEN REVERSED AFTER MAINT COMPLETION. PROCS WERE REVERSED; THE ACFT WAS TEST PRESSURIZED WITHOUT ANY FURTHER PROBS; AND RETURNED TO SVC.

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ACFT DEPARTED ON FIRST FLT AFTER HVY MAINT VISIT FOR APU CHANGE AND AFT PRESSURE BULKHEAD DAMAGE REPAIRS. ACFT WOULD NOT PRESSURIZE AFTER TKOF. FLT RETURNED TO GATE AND WAS TAKEN TO HANGAR. AFTER TROUBLESHOOTING; OUTFLOW VALVE WAS FOUND BLOCKED IN OPEN POS. ACFT WAS FOUND TO HAVE DEPARTED WITH IMPROPERLY CLRED MEL FOR OUTFLOW VALVE. WHEN ACFT ARRIVED FOR HVY MAINT VISIT; OUTFLOW VALVE WAS ON MEL. WHEN VISIT WAS COMPLETE; MEL APPEARS TO HAVE BEEN IMPROPERLY CLRED WITH OUTFLOW VALVE LEFT OPEN. MEL WAS PROPERLY CLRED; PRESSURIZATION SYS OPERATED NORMALLY AND ACFT RETURNED TO SVC. I FEEL COMPANY NEEDS BETTER CTL; TRACKING; AND CLRING PROCS FOR MEL OPS. MY INVOLVEMENT IN THIS INCIDENT WAS AS INSPECTOR IN HVY MAINT. THIS PARTICULAR PRESSURIZATION ITEM WAS NOT A REQUIRED INSPECTION ITEM PER OUR GENERAL MAINT MANUAL. IF IT HAD BEEN; 'ANOTHER SET OF EYES' WOULD HAVE BEEN AVAILABLE; POSSIBLY PREVENTING THIS FROM HAPPENING. SUPPLEMENTAL INFO FROM ACN 326175: I WAS ASSIGNED TO TROUBLESHOOT AN ACFT WHICH HAD RETURNED TO THE FIELD WITH A PRESSURIZATION FAILURE. IN MY BRIEFING IT WAS RPTED TO ME THAT THIS ACFT MAY HAVE BEEN RELEASED WITHOUT CERTAIN PROCS ACCOMPLISHED TO ALLOW IT TO PRESSURIZE. ALSO RPTED TO ME WAS OTHER MECHS FROM DAY SHIFT ATTEMPTED TO PRESSURIZE AND WERE UNABLE. ONCE AT THE ACFT I REVIEWED THE ACFT LOGBOOK AND TOOK NOTE OF AN MEL PROC ACCOMPLISHED IN ORDER FOR THE ACFT TO BE MAINT FERRY FLTED FOR REPAIRS AND THAT NO ENTRY SHOWED THOSE PROCS REVERSED. AS PREVIOUS MECHS HAD ALSO LOOKED AT THE OUTFLOW VALVE AND FOUND A PIECE OF RUBBER HOSE IN L-HAND OUTFLOW VALVE AS PER MEL PROCS; I ALSO CAREFULLY EXERCISED BOTH OUTFLOW VALVES AND UNDER LOWER SECTION OF L-HAND OUTFLOW VALVE NOTED SMALL BLACK BLOCK OR HOSE WHICH I REMOVED. I NOTED NO OTHER DISCREPANCIES. I THEN STARTED THE APU AND PROCEEDED TO PRESSURIZE THE ACFT. ON THE MULTIFUNCTION DISPLAY UNIT THERE WERE NO WARNINGS OR INDICATIONS OF A PROB. I PRESSURIZED THE ACFT TO 4 PSI AND MAINTAINED THAT PRESSURE. AS NO OTHER HISTORY HAD INDICATED PROBS PRIOR TO THE MEL; I CLOSED PANELS THAT WERE OPENED TO INSPECT OUTFLOW VALVES AND MADE APPROPRIATE LOGBOOK ENTRIES TO RELEASE ACFT FOR SVC.