Your problem isn't the OUTLET of the lift pump. If it were, it would be leaking fuel, externally (make a mess), while the lift pump is running. The problem is at, or prior to, the INLET to the lift pump. Pressurizing the tank may help find leaks that may not show up otherwise, but if you have leaks between the lift pump and injection pump, they should show while the lift pump is running. So, if you find leaks between the lift pump and injection pump, fix them, but they are NOT the source of air in your system.