Hy HFfarms, I am with sweepleader, as I have never encountered the line setup you have on your master cylinder. As you showed in your pictures the apply line from the master enters the booster close to the diaphragm, the booster output line is at the end of the cylindrical projection at the end of the booster, the output line is usually a single line leading to a tee on the frame which leads to the front and rear brakes on a single chamber system. By adding the second line from the booster output back to the master cylinder you are sending boosted pressure fluid back to the master cylinder and making a big loop with some apply fluid and some boosted apply fluid. If it were my truck I would remove the second line back to the master cylinder and plug the second outlet on the booster output, and see how the brakes function, hope that helps.
