Okay. I'm not an expert, but I don't see how the fuel can siphon out of the Rochester Carb normally found on the older 235s and 261s. Where the fuel goes into the carb is on the top of the carb. If the float is up, the seat would be closed, if is open because of low fuel level in the carb, air would enter the fuel line instead of fuel.

Most likely culprit on loosing fuel I would think would be the carb leaking somewhere and emptying out, or the fuel evaporating out of the bowl after it sits a while.

Also, if there is supposed to be a check valve in the carb for the accelator pump it could have been left out on the rebuild and not pumping the fuel into the manifold.

I don't ever remember seeing the glass sediment bowl on my truck completely full, yet the truck runs fine.