I have a 2004 f150 5.4 and I am having issues with my brakes. I had a vaccum line behind my intake come out on me, so I had to remove my intake to get that line back on. About a week after this happened, my brake pedal started going soft. I can still stop good, but its soft. I drove it like this for the past 6 months.

2 weeks ago I replaced my rear calipers and pads, bled the whole system out, pedal feels the same. So I ordered a new master cylinder, blench bled that, installed it, and bled the whole system.......pedal still feels the same.

I can pump the pedal with the truck off and it gets hard, but as soon as I turn the truck on, the pedal turns soft. Like I said, I can still brake decent, but the pedal almost goes to the floor. What could be wrong????? Loosing my mind!!!

 

I have a similar issue with a soft pedal. You mention you've replaced everything except the front calipers. That is where I plan to start as it's where most of the braking is done and the most likely place for an issue imo.

 

There's a sticky about soft brake pedal. Have you looked at that?