I was having the soft brake pedal issue running a dual diaphragm booster, 1 inch m/c, and four wheel disk. I read an article on the interwebs about having a round turn in the brake lines that was above the m/c connections. They stated that air could be trapped in the loop and would be very difficult to get out, so I replaced the lines and formed the brake line to make a 90 degree down turn to the front and rear connections. That solved the soft brake pedal.
I had a 1 1/8th m/c on the car and it would not stop the car without a really heavy foot. hope this helps someone!