[eggsuckindog]

Here is the ticket - Isolate and Eliminate
Its not a motor issue period - forget that
Now with the bulb being sucked flat - its either the bulb - the tank pickup - an anti syphon valve or the vent.
Do one at a time so you isolate and eliminate each item - first when it sucks the bulb flat open the tank fill - you get gas its the vent, you just vented it.
Next - if thats not it when it sucks flat blow back through the line - that will clear the tank pickup, should be easy to blow and you should here bubbles
The anti syphon valve is in the tank fitting and yes they do cause issues, personally I would take it out and use a straight fitting but I'm a rebel.
The bulb itself can be checked by getting the motor started and warm, shut it down and remove the bulb itself and try it.
If none of that works try bypassing the filter

Its between the tank and the motor somewhere - I don't like the 1/4" line though should bigger IMHO but not the issue I don't think

That will keep you from buying anything before you find out what it is - isolate and eliminate - hope this helps