I know this is an old thread but a BiG thanks for taking the time to post these solutions. I have had my tractor lose prime twice in the last month and I am pretty sure it is from letting the fuel level get too low which is directly relate to my faulty fuel gauge!
Anyway, at least I now know why it's is happening and can hopefully prevent it going forward. I did not see where there was a check valve on my fuel line, so either it was removed (I bought this used) or I totally missed it. If this happens again, I plan to install the check valve and replace the fuel line. I may install the electric pump if it does not require a high degree of skill.