Brake problems

I have a 1995 F-350 crew cab diesel. It is a 4wd. My problem is the calipers keep locking up. I have replaced the calipers and the rubber brake lines. After that was unsuccessful I followed the lines and kept cracking open fittings to see where the problem was that I could not pry the calipers open anymore. The result is I keep going back to the master cylinder but the master cylinder is not bad, it is in fact brand new. I also adjusted the nut in the brake booster thinking that was maybe causing the pistons in the master cylinder not to fully open back up to allow the fluid to flow back. Any help you could give would be greatly appreciated.

Sorry, but it sound like a bad master cylinder from your info. Does the vehicle have ABS?