My 1983 Jimmy has a 6.2L diesel. Frequently, not all the time, my brakes seem to build pressure and begin to lock up as I drive. Front mostly.. I have replaced the calipers, rotors, pads, hoses and master cylinder. It still does it. Could it be the Hydra-Boost leaking internaly? Any ideas?

Never had the problem, but it would appear pressure is being released thru the hydo boost to push on the master cylinder piston. Normally the pressure occurs after you push on the brake pedal. The valve in the hydro-boost has probably gone bad. I assume the lockup happened before you did any work on the vehicle.