Hi Bryan. Dont know if this is the same problem. I have a similar vehicle in at the moment, a petrol vectra of some sort, cant remember details. Below 20L fuel if runs rough, holds back, etc, with no faults present in ECU and live data all ok. Take it pretty much on the 20L and its suddenly perfect. Ive been along the lines of the pump but no joy so far, took the pump out and it pressurizes with just the pickup filter in fuel, so thats not faulty. And as you quite rightly say, the pump chamber cant be sealed, how would it get fuel in? They're normally just to dampen the movement of the fuel.