[dsraven]

if you wanna check to see if it is fuel bleeding off and siphoning the float bowls empty just try putting a line clamp on the soft rubber fuel line after you run it and turn it off.let it sit for however long it usually takes for it not to start, then try to start it. if it starts like it should then it is siphoning likely. another way to quickly check is to let it sit for the normal time it takea to not start, then take off the air filter and look down inside the throat of the carb while you pump the throttle. you should see fuel squirting out of the accelerator pump nozzles.
I have had many brand new orr rebuilt mechanical fuel pumps over the years and some will have the valves stick open or closed so no fuel is allowed to pump. theoretically the valves in the pump should not allow fuel to run back to tank but we have all seen it not be so.I have had more issues with the three line pumps than the two line ones.
I have seen some cars/trucks where guys have used an electric pump, run in parallel with the mechanical pump, so they can prime the system before attempting to start the engine. it would be on a switch so you would knowingly have to turn it on or off.