When I bought my 2003 6.0l CCSB, which my kids dubbed The Hulk, it had a failed rear brake caliper and a soft pedal. My goal for the truck is to tow a fifth wheel, so I put drilled and slotted rotors and heavy duty pads on all four wheels when I replaced the rear caliper.
However, I couldn't get the pedal softness solved, no matter how many time I bled the brakes. I didn't think it was th master cylinder, as I could get the pedal to hold at about half depth with pumping the brakes. I did think that maybe there was a bubble trapped in the master cylinder, so I read up on bleeding the master cylinder in the vehicle.
One suggestion I saw was to make sure the cylinder was level so the bubble would go up into the reservoir. It made more sense to me to make sure the cylinder is nose down to force any air back to the piston, and you can then sweep it up into the reservoir by pushing the pedal slightly forward.
I parked the truck at a severe nose down angle and move the pedal slightly several times. There didn't seem to be any improvement. I was about to give up, but I figured I might as well flush some more fluid through the brake lines while it was nose down and easy to crawl under. Well you would not believe how much air came out of the rear lines, and now I have a firm brake pedal.
I would be interested to know if this works for anyone else. My firm pedal has lasted for more than four hours. Should I call a doctor?