DC10 CREW WAS UNABLE TO PRESSURIZE ACFT AFTER TKOF. CREW DUMPED FUEL AND RETURNED TO DEP STATION. LNDG WAS 2300 LBS OVERWT.

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ON TKOF; ACFT PRESSURIZATION PROB OCCURRED. I WAS PNF AND ADVISED SO MY EARS HURT; WAS THERE A CABIN PROB. SHORTLY THEREAFTER; FLT ATTENDANTS CALLED INFORMING THAT 4R DOOR; THEY COULD SEE DAYLIGHT AROUND TOP AND 2R DOOR WAS NOISY. THE CAPT (FLYING) SLOWED ASCENT RATE AND LEVELED OFF AT 3300 FT. I INFORMED DEP CTL OF A PROB WITH CABIN PRESSURE AND DEP HAD US DSND AND MAINTAIN 3000 FT. WE WERE GIVEN VECTORS AT 3000 FT AND CLRED TO DUMP FUEL WHEN IT BECAME CLR THE FE COULD NOT RECTIFY THE SIT WITH MANUAL OR STANDBY CTL. FUEL DUMP BASED ON CALCULATIONS BY FE FOR 1.5 MINS; TO BRING ACFT UNDER MAX LNDG WT. WE WERE GIVEN VECTORS FOR APCH AND LNDG BY APCH CTL WHEN FINISHED WITH FUEL DUMP AND COMPANY NOTIFIED. APCH AND LNDG NONEVENTFUL (SMOOTH). AFTER LNDG AND MAINT PERSONNEL WORKING ON THE PROB; DISPATCH NOTIFIED US THAT THE ACFT HAD IN FACT LANDED OVERWT AT APPROX 368000 LBS AS OPPOSED TO 363500 LBS. RECALCULATING WT SHOWED 365800 LBS OR 2300 LBS OVER MAX LNDG WT. OVERWT LNDG CHKS PERFORMED BY MAINT AND ENTERED IN LOGBOOK. IN THE FUTURE; GROSS WT ON PERFORMANCE PAGE COULD BE USED TO CONFIRM OR XREF THE CALCULATED LNDG WT. ALTHOUGH WITH THE POSSIBLE DOOR PROB AND LIMITED TIME PRIOR TO APCH PRECLUDED A LOT OF EXTRA TIME AVAILABLE FOR XREFING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT DURING MAINT TROUBLESHOOTING THEY FOUND A JAMMED OUTFLOW VALVE. ONLY BY USING EXCESSIVE FORCE ON THE MANUAL OUTFLOW CTLR WERE THEY ABLE TO BREAK THE JAM. AFTER FREEING THE VALVE; MAINT REPLACED THE AUTOCTLR PRECAUTIONARY. THE RPTR FURTHER STATES THEY WERE FLYING WITH AN EXCEPTIONALLY WEAK SO. THIS SO HAS A REPUTATION OF BEING SO INCOMPETENT THAT OTHER CREW MEMBERS ARE CARRYING THE SO. THE SO FAILED TO CHK THE CTL VALVE FOR FREEDOM ON COCKPIT PREFLT. THE CAPT ELECTED TO RETURN TO THE DEP STATION RATHER THAN ATTEMPT INFLT TROUBLESHOOTING BECAUSE WHEN THE SO IS CHALLENGED WITH UNUSUAL SITS; THE SO BREAKS DOWN AND CRIES. THE SO THEN NO LONGER PARTICIPATES AS A CREW MEMBER. THE SO WAS RESPONSIBLE FOR THE MISTAKEN OVERWT LNDG.