When the collector cell is full, the level control valve closes, thus stopping the supply of fuel from the other 3 cells. This is the only way to stop the flow of fuel into the collector cell as the booster pumps cannot be selected off in flight. As the Collector cell empties, the Level Control Valve opens to allow the Collector cell to be replenished.
This is intended to ensure that the Collector cell will be the last one to run out of fuel.
Great! That's the answer to my question. Thank you very much, sir! Don't worry about currency on the aircraft...it seems it's the very same aircraft you last saw 15 years ago
(It's perfect as it is IMO)
And yes, I'd like that schematic for further understanding of the system. Much appreciated!