ACR WDB UNABLE TO PRESSURIZE; RETURN AND LAND MANDATED.

Narrative

ACFT WAS DISPATCHED WITH LWR. CTR AVIONICS COMPARTMENT DOOR OPEN. DOOR WAS IN LWR. AREA OF TRACK AND LOOKED IF IT WERE CLOSED. (GAP APPROX 1 INCH WHEN DOOR LOWERED ON TRACK). CREW DID NOT RPT OF ANY DOOR WARNING LIGHTS WHEN DISPATCHED. AFTER THEY WERE AIRBORNE; CREW CALLED BACK AND ADVISED MAINT THEY COULD NOT PRESSURIZE ACFT. IT WAS NECESSARY TO DUMP FUEL DUE TO ACFT WT FOR LNDG WOULD BE EXCEEDED. CREW ADVISED MAINT THAT R-HAND FUEL DUMP VALVE WOULD NOT OPEN. NECESSARY TO DEFINE R-HAND AND L-HAND FUEL DUMP VALVES. SOME OF THE REASONS FOR THIS INCIDENT HAPPENING WAS A LOT OF ACTIVITY (MAINT WISE) PRIOR TO DEP OF TRIP. ACFT WAS TAXIED TO GATE FROM REMOTE PARKING AREA. THE TAXI CREW ENTERED ACFT THROUGH THIS DOOR TO GET INTO ACFT. WHEN TAXIED TO GATE MAINT TAXI CREW SHIFT ENDED AT 0700 (MIDNIGHT CREW). DAY SHIFT TOOK OVER AND MAINT WAS ADVISED OF SEVERAL ITEMS DURING CREW PREFLT (TRIM WARNING AND OIL SVC; OVENS; FULL GALLERY; ETC). NO MAINT WAS ACCOMPLISHED IN CTR AVIONICS COMPARTMENT. DOOR WAS NOT NOTED TO BE OPEN ON SO'S WALK AROUND OR BY MAINT DISPATCH MECH.