John;

You are absolutely right, but I have seen where one of those valves was completely shot, but the car (Toyota) ran and started without too much strangeness (it was a carburatet car and - from memory - it was I believe the output valve which crumbled apart)...the way I reconciled this is that the valves are essentially two check-valves in series, with a diaphram variable volume in between, allowing flow in ONLY one direction...to the carb!...so if the output valve goes away, and the diaphram decreases the volume, I figured the pump flows toward the carb, when the diaphram then changes direction and increases the volume, it was somehow easier for fuel to flow from the (vented?) tank through the input valve than it was for fuel to get sucked back from the (sealed?) bowl.

Point is that....wouldn't both valves have to be open to flow to allow drainage back to the tank?

Interested in the answer too!