That "fuel reservoir" Is also called the 6 port reservoir switching valve. It is before the High pressure pump, and has 2 lines to each tank and 2 to the engine, 1 supply and 1 return each. When that Valve fails, you get 'cross flow' which is mostly return to the wrong tank, but can be pumped in from the other tank. Check the wiring to the rear sender (low pressure), and if you think the rear tank is contaminated, get that gas out. The cross flow means you need a new selector valve, but if you have no intention of using the midship tank, you can bypass it.
-Mike