Q: Car keeps breaking down

What I believe has happened is that the mechanic you went through lied to you. Based on his directions for you after the fuel pump replacement, it sounds like the original fuel pump was actually the correct size and that he installed one that is too small for your vehicle. Evidence to support this statement would be the fact that he said to only allow the fuel to go down to half a tank. If the fuel pump is too small for your fuel tank, the pump would not be able to reach the fuel once the level has dropped below half tank.

Also, having to prime the fuel pump with your key each time you start the vehicle is another indication that the fuel pump is not the correct pump for your vehicle. It is possible that your vehicle may have two different fuel tank options. I would inquire about this before replacing the fuel pump to ensure that the correct parts are installed in order to save you time and money this time around.

If this turns out to be a correct diagnosis, I would recommend taking your vehicle back to the mechanic who originally replaced your fuel pump with the invoice from the final repair. Accuse the mechanic of a misdiagnosis and request that he or his company reimburse you for your expenses.