If there's no fuel in the line after sitting or when you run it out of gas, you have to prime the carb. The bowls have no fuel in them. The starter cranking will not be enough to prime the fuel pump but when you start the truck up by pouring a little gas in the top of the carb, it will prime the whole system quickly.
I have been doing this a lot recently after the engine rebuild, replacing old lines and such.
