Bison has it right. You have an air leak that is allowing th fuel to return to the fuel tank. Many times you can have a leak that will allow air into the system but not leak fuel out of the system. The push type of primer pumps are real bad about leaking air.
I would just look all the fuel lines over real close, looking for cracks in any rubber lines or lose fittings on steel lines.
I have ran into ones that I never could find the air leak. I would then install a check valve at the fuel tank. This would let fuel go to the engine but not run back to the tank.
A way to think about it is this way. IF you stick a hose down into a liquid, then hold your finger real tight over the end and lift the hose out of the liquid. You finger has the weight/vacuum trying to pull the liquid back out the hose.
This small amount of vacuum is how your fuel system is draining back into the tank. Some where you are getting air into the top of your system and it is allowing the fuel to run out of the lines.