Primer Bulb Vacuum Issue - 05 Aug, 2019

Have a re occurring issue where my primer bulb is getting a vacuum, sucking flat and starving the engine of fuel. I have replaced the fuel lines, filters and primer bulb. When I use an external tank I have zero issues, once I re connect to internal tank the issue continues. If I run 5-10mpg, it doesnt seem to happen, soon as throttle up to 20mpg it happens quickly. Any other ideas on what could be my issue? I have also tried unscrewing my fuel cap when the issue occurs and still nothing. I am open to any suggestions, thank you.
-Brackish_B