B747-400 Captain reported a faulty fuel boost pump caused suspicion of a fuel leak enroute; forcing an air return.

Narrative

Our crew was originally scheduled to operate a flight from ZZZ to ZZZZ on Day 0 using Aircraft X; a B747-400F series aircraft. Standard preflight; no relevant MEL's active. I do specifically remember accomplishing the 'Fuel Distribution Check' during preflight and noticing that main fuel tanks 2 and 3 were appropriately loaded and nearly evenly matched. FMC PERF page 'RESERVES' pilot entry was set to the regulatory minimum to the best of my recollection. That value would have been the fuel burn from filed destination to alternate airport; plus 1.5 times the destination reserve fuel as listed on in our departure paperwork. We have no way to keep a copy of that paperwork after the flight ends however; so I cannot provide a specific number. Fuel system configuration for taxi and takeoff was fuel override pumps supplying all engines; center tank pumps off; and main tank pumps in standby. Start / Taxi / Takeoff uneventful. Shortly after takeoff we received the EICAS advisory message 'FUEL IMBAL 2-3'. We acknowledged the message and stated that we would address it later in the departure as our workload allowed.XA38 (5 minutes elapsed): At this time we began the QRH checklist for FUEL IMBAL 2-3 and paused at step X which is where we analyze indications to determine if an engine fuel leak is suspected. Displaying the fuel synoptic revealed a 6;200 pound split between main fuel tanks 2 and 3 and passing through approximately 11;000 feet. Tank 2 was the low tank.XA40 (7 minutes elapsed): Per the checklist we noted all engine fuel flow indications matched and normal. We were on an ATC assigned direct leg to a down airway waypoint; so we hadn't yet passed a flight plan fix to get a fuel remaining vs. fuel planned check. I didn't manually enter an FMC fix to get an abeam waypoint fuel check since we were getting close to our direct-to waypoint anyway.XA42 (9 minutes elapsed): We noted FMC PROG page 2 TOTALIZER quantity 207.0 and CALCULATED quantity 207.6. TOTALIZER was in use as directed by AOM normal procedure. Fuel imbalance now 7;400 pounds. At this point we had conclusively satisfied the checklist step X condition 'On PROGRESS page 2; the totalizer is less than the calculated fuel.' This also conclusively satisfied the checklist step 3 that 'A fuel leak should be suspected.' Continuing with checklist item #Y and with an engine fuel leak suspected; we leave the FUEL IMBAL 2-3 checklist and proceed to the Fuel Leak Engine checklist.XA50 (17 minutes elapsed): Totalizer fuel 202.0 and Calculated 202.5. Fuel imbalance 8;200 pounds. Fuel Leak Engine checklist steps 1-6 initiated about this timeframe. No sustained increase in imbalance noted during about 5-10 minutes (guess) that we initially monitored; so based on that; I didn't believe that a greater than 1;000 pound imbalance would occur within the 30 minute period. Paused the checklist at that point while we were in the 30 minute monitoring period. Looked ahead at the rest of the checklist and did not anticipate confirming an engine fuel leak. Completed the Fuel Leak Engine checklist. No visual check possible due to nighttime conditions. All engine fuel flows equal and normal. Conclusion was no engine fuel leak yet indicated. Step Z - Engine fuel leak is not confirmed - Go to step A - FUEL DISAGREE message is not shown - Go to step B - FUEL QTY LOW message is blank - end of checklist.XA58 (25 minutes elapsed): Fuel check at waypoint ZZZZZ; POS fuel 200.1. I believe at this point the waypoint fuel was noted to be slightly less than planned; and therefore satisfied FUEL IMBAL 2-3 checklist item X with a second condition to suspect an engine fuel leak 'The total fuel remaining on EICAS is less than the planned fuel remaining.' So now we had two data points to indicate that an engine fuel leak is suspected. Still enroute to destination. I believe that somewhere in this timeframe we spoke with Dispatch and Maintenance Control. Maintenance Control didn't have input regarding the imbalance and I suggested a return to ZZZ with Dispatch and they agreed. Dispatch issued an Amended Release for RTB (Return to Base) to ZZZ. Out of abundance of caution and in the interest of safety; I advised ATC and specified that no priority or assistance were requested. This was primarily because we hadn't yet resolved the imbalance; and I didn't know if there was still the possibility of leaking fuel to develop later (fire risk). With previous events such as severe fuel imbalances developing enroute and fuel leaks turning into engine fires; and having a QRH open to pages looking for fuel leaks; I just felt that it wouldn't hurt to have the latitude to address our situation as provided by the QRH Additional Instructions guidance. I didn't want to end up deviating from established / expected procedure erroneously and cause more trouble for anyone.XB04 (32 minutes elapsed): Total imbalance 10;500 pounds. Tank 2 is still the low tank. Air turnback to ZZZ. I had firmly believed that the INSUFFICIENT FUEL message had displayed by this point; but the other crewmembers have confirmed that it did not. The message only came on after we began the air turnback; which makes sense since the flight plan was way off at that point. So that proves that my recall is flawed on the matter; and I would please ask that you would disregard any previous statement of mine that it happened prior to turnback. I was wrong about this and I apologize.XB21 - XB40: Normal fuel balancing in progress. Proceeded to ZZZ and entered holding at ATC request until ready for the approach. Once fuel was balanced; we landed in ZZZ; stopped on taxiway; and shut down #1 and 2 engines on left wing to have airport rescue inspect the left wing and engines for signs of a fuel leak. No leaks were detected; so we restarted #1 engine and taxiid back to the ramp. Local maintenance was unsure of the cause of the imbalance initially. They had to dip stick the tanks to confirm correct fuel readings in the cockpit. Post flight analysis of the fuel card download revealed faulty #3 tank override pump(s) which caused reduced pressure in the right manifold. The left override pumps therefore supplied fuel flow to all engines; causing tank 2 to drain faster than tank 3.With the benefit of hindsight and having a better understanding of the checklists now; I also understand that I should not have turned the override pumps back on after completing the Fuel Leak Engine checklist. Having not been able to detect an engine fuel leak; yet still having two of the suspected leak conditions listed at the end of the Fuel Leak Engine checklist; I just wasn't thinking that the override pumps in tank 3 would have been bad. All the pumps on the synoptic showed green (normal pressure); so I thought the best thing to do was to return to normal configuration until we could begin the fuel balancing process. I see how that thought process was wrong now; and the override pumps could have just stayed off as the checklist directed. It wasn't my intention to deviate from procedure; but I was also trying to do the best course of action as I was able. A case study recently discussed had a scenario regarding the use of wing anti-ice where the Electronic Checklist is overridden and disregarded depending on the phase of flight / aircraft configuration; flaps retracted during go-around with left WAI (Wing Anti-Ice) valve failed open. Having this case study on my mind; I thought our situation could be similar; but I think now that the checklist was satisfactory as-is; even with the faulty pump(s).I am submitting this report in case I have deviated from prescribed procedure or improperly executed a non-normal checklist. It is never my intention to do so; but in situations like this I often find that my recollection is not accurate. Thank you for your time.