something has to be wrong with your fuel routing if you are getting fuel pressure on the return line.
fuel has to go to the fuel pump, then to filter, then to IP, then to Injector and what is not used at the injector then gravity drains back to the fuel tank through the return line. The return lines are not designed to receive pressure.
check that the output of the fuel pump is going to the metal line that goes to the fuel filter and not the return line connection back to the tank.
You need to check to see that you actually have fuel pressure after the fuel pump. You may just have bad fuel pump. The pumps can still move fluid when they go bad but will not put up enough pressure to push it through the filter.