RETURN LAND ACCOUNT UNABLE TO PRESSURIZE ACFT.

Narrative

ON CLBOUT FROM BUR ENRTE TO OAK WE WERE UNABLE TO PRESSURIZE THE ACFT AND RETURNED TO BUR. FLT DURATION WAS ABOUT 10 MINS AND MAX ALT WAS 7000'. MAINT PERSONNEL MET THE ACFT AND FOUND THE FORWARD CARGO DOOR NOT COMPLETELY CLOSED. ON THE MASTER WARNING PANEL THERE IS A YELLOW CAUTION LIGHT: 'LOWER DOOR NOT CLOSED' THAT SHOULD HAVE BEEN ILLUMINATED. UPON FURTHER INSPECTION WE FOUND THAT BOTH PEANUT BULBS FOR THAT CAUTION LIGHT WERE BURNED OUT. AT 1 HR PRIOR TO OUR DEP TIME THE F/O AND I STARTED AN ORIGINATING PREFLT ON THIS ACFT. THIS PREFLT IS EXTENSIVE AND REQUIRES CHKS OF ALL SYSTEMS ON THE ACFT; INCLUDING THE LIGHTS ON THE MASTER WARNING PANEL. DURING THIS PREFLT WE FOUND ALL THE LIGHTS ON THE MASTER WARNING PANEL TO BE FUNCTIONING PROPERLY. THE CARGO PIT DOOR IS NORMALLY CLOSED BY GND SVC PERSONNEL JUST MINS PRIOR TO DEP; ONCE THE FINAL BAGS ARE LOADED. IT APPEARED THAT THEY HAD TRIED TO CLOSE THE CARGO PIT DOOR; HOWEVER ONE OF THE LOCKING PINS IN THE DOOR MECHANISM HAD NOT PROPERLY SEATED ITSELF.