DC9 RETURN LAND AFTER FUEL IS NOTED TO BE SIPHONING OUT OF THE L WING. CREW ATTEMPTS TO USE THE XFEED CHKLIST AND CREATES A FURTHER FUEL IMBAL. THE R AFT MAIN BOOST PUMP HAD BEEN DEFERRED ON THE MEL.

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FLT ATTENDANT NOTIFIED ME OF A STREAM OF FLUID COMING OUT OF L WING. I HAD AN FAA INSPECTOR ON JUMP SEAT AND I ASKED HIM TO TAKE A LOOK. HE RPTED THAT A HVY STREAM OF FUEL WAS LEAKING FROM OUTBOARD OF L WING. ACFT FUEL BURN WAS OK; BUT TOTAL FUEL WAS ABOUT 600 LBS LESS THAN IT SHOULD BE. I SUSPECTED UNCTLED FUEL XFER WAS OCCURRING. ACFT WAS LEVELED OFF. A RETURN TO DFW WAS INITIATED AND AN EMER WAS DECLARED. CREW OPERATING MANUAL PROC WAS FOLLOWED BUT WHEN XFEED WAS OPENED; A FURTHER FUEL IMBAL OCCURRED. ON LNDG; HAD A 1000 LB FUEL IMBAL. DISPATCH WAS NOTIFIED INFLT OF SIT. ENTIRE CREW PERFORMED ADMIRABLY. LANDED UNEVENTFULLY. ACFT WAS DISPATCHED WITH R AFT FUEL BOOST PUMP INOP. ALSO; A KNOWN FUEL LEAK ON L WING WAS NOTED IN LOGBOOK AND SIGNED OFF AS OK FOR FLT. THE FACT THAT THE R AFT BOOST PUMP WAS INOP MADE THE CREW OPERATING MANUAL PROC DIFFICULT TO COMPLY WITH SINCE IT MADE THE FUEL IMBAL GREATER WHEN THE XFEED VALVE WAS OPEN. THE FACT THAT WE HAD A KNOWN FUEL LEAK AND A R AFT BOOST PUMP INOP PLUS RPTED HVY STREAM OF FUEL LEAKING/VENTING PLUS 2 HR FLT AHEAD MADE ME RETURN TO DFW. DURING THE RETURN TO DFW I REQUESTED AND ENCOURAGED INPUT FROM THE FAA INSPECTOR IN THE COCKPIT JUMP SEAT. I RECEIVED NO INPUT ONE WAY OR ANOTHER FROM HIM. AFTER LNDG; THE MAINT SUPVR AT DFW RESEARCHED THE HISTORY ON THIS AIRPLANE AND FOUND NUMEROUS EVENTS OF PREVIOUS FUEL XFERS. HE HIMSELF OBSERVED AND POINTED OUT TO ME A XFER OF FUEL AT THE GATE AFTER WE PARKED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT HE ATTEMPTED THE XFEEDING INITIALLY SINCE HE THOUGHT HE HAD FUEL XFERRING FROM THE CTR TANK AND HE WASN'T CERTAIN AS TO 'WHAT HE HAD' IN THE WAY OF A SIT. ALL PUMPS HAD BEEN ON SINCE THE BEFORE START CHKLIST AS THAT IS COMPANY PROC; ALL TANK BOOST PUMPS ON AND LEAVE THEM ALL ON; L; CTR; AND R. NOW; WITH THE XFEEDS OPEN; HE WAS PUMPING FUEL FROM THE L TANK TO THE R; OVERFILLING IT. THE FUEL THEN WENT TO THE SURGE OR VENT BOX IN THE R WING; THEN FLOWING OUT THE OVERFLOW EXIT LINE LOCATED ON THE L WING. THIS ACTION EXACERBATED THE PREVIOUS CONDITION. THAT CONDITION WAS FUEL BEING PUMPED FROM THE CTR TANK TO THE R TANK THROUGH A FAULTY '1 WAY' POPPET VALVE THAT IS SUPPOSED TO PREVENT INADVERTENT FUEL XFERS. SO THE FUEL WAS TAKING THE SAME COURSE AS BEFORE BUT SINCE THE R BOOST PUMP PRESSURE WAS LOWER THAN THE L; THE FUEL FROM THE L TANK WAS BEING PUMPED INTO THE SURGE TANK AS WELL AS THE FUEL FROM THE CTR TANK; AND THEN OVERBOARD. A SORT OF 'DOUBLE WHAMMY.' THE RPTR SAID THAT THIS CHKLIST PROC IS FOR ALL PUMPS WORKING BUT THE CHKLIST DOESN'T SAY THAT. HE FEELS THAT A NOTE OF WARNING ON THE CHKLIST SAYING 'DO NOT USE IF ONE PUMP IN L OR R TANK IS INOP' WOULD BE APPROPRIATE. HE ENDED UP TURNING OFF ALL PUMPS IN DSCNT EXCEPT 1 LEFT PUMP; TURNING THEM ALL BACK ON AT 5000 FT. HE HAD DECLARED AN EMER FOR THE DSCNT. IT TURNED OUT THAT THE PREVIOUS WRITE-UP REGARDING FUEL LEAKAGE WAS NOT ASSOCIATED WITH THE EVENT; AS IT WAS MINOR UNDER WING SEAM LEAKAGE THAT WAS 'IN LIMITS' AND SET UP FOR REPAIR. AS A SIDE NOTE HE STATED THAT THE OTHER CARRIER HE HAD BEEN WITH USED ONLY THE R AND L MAIN BOOST PUMPS UNTIL IN CLB; THEN SHIFTED TO THE CTR TANK FEED.