You can only purge air from the injector lines by cranking the engine over, either by you or by someone else. It doesn't matter. I'd crack all 8 lines at once. One at a time will take 8 times as much cranking, putting 8 times as much strain on your starter and batteries. Since you said that you have about 129,000 miles on the original injector pump, it may be time to replace it with a good quality one. Purging the air will only help you until you shut your truck off and the fuel drains back out of your fuel lines. They are saying that you have air intrusion into the fuel system somewhere. If you don't find and fix that, you'll always be getting air into your fuel system. Having 1/2 tank of fuel is fine. The shower head problem happens at about 1/4 tank. This is when the end of your fuel pickup in the tank (known as a "shower head" because of it's shape) falls off onto the bottom of the tank. This means that you can't draw as much fuel out of your tank since the pickup is now shorter. You can only get to about 1/4 tank.