If the return lines are leaking air in, they will certainly leak fuel out. It takes a bit of running to purge all the air out of the injectors and return circuit, but it shouldn't be much (essentially the amount of air in the lines), and it shouldn't take too long. If the engine was ran only minutes to an hour, there may still be air in the return from the injector and return line replacement. Otherwise, air coming out of the IP, is air that got into it. If you have no external fuel leaks, and the lift pump is working and healthy, then look for a leak between the lift pump and tank sender unit, or the sender unit itself.

The mechanical fuel lift pump has a weep hole at the bottom of the pod. When the pump fails, it will leak fuel from there. If fuel is leaking from the weep hole, the pump has failed due to the ruptured diaphragm. This is often misdiagnosed as a loose line at the pump (and folks overtighten the nut until it strips or breaks). If it has failed, it may still pump enough fuel to keep an engine running well, at least for the moment, but may also allow air into the system (the fuel leaking out is traded for air leaking in).