I see how old this thread is and perhaps someone who has solved this problem can comment, but I thought I would mention that I have a similar issue. Didn't replace calipers, brake pads, or adjust anything. I had to replace a stripped junction block (the piece connected to the rear brake hose that connects to the axle and splits the rear brake lines) The only parts replaced were the brake hose, flare nuts, and eventually the wheel cylinder. My brakes are currently stiff at 50% travel, then sink slowly. E-brake is tight.
This tells me that my issue and possibly some of the others here is an incompletely bled brake system. Just a couple notes...
1. The brakes should be bled from the bleeder valve furthest from MC.... that would be pass, rear.
2. After reading some previous posters, it may be necessary to have the truck pitched so the front end is higher to accomplish proper bleed? Just a thought.