Not sure if it helps or not, but when this happened last and i pulled the main fuel line at the inlet of the fuel pump it was dry... gas did not flow out. when i hooked the mityvac up to the end i easily got a strong flow of gas in short order (about 3-5 s of pumping)... could this be an issue of air entering the main line somewhere and it correcting itself when I crack the fuel lines at the carb or fuel pump?

my main fuel line is 3/8 inch and i am runnning a 1/4 return line and factory type fuel seperator

thanks