CJ,
At last a fellow engineer to bounce off!
The check valve is fitted just above the the pump, when the pump is on the valve lifts of its seat.
The valve returns to its seat when the pump is off and the other pump is turned on.
Now my experience with check valves is they usually stick off their seat ( the culprit usually being dirt or sediment ) allowing fuel to flow back through the valve.
Are you saying that in your experience, a valve would normally stick on its seat and prevent flow at all?