69 k2500 4wd disc brake conversion problem

[smoke308]

First time to post. Have a braking problem on my 69 GMC 2500 4wd. Converted front brakes to disc with a kit from RAM Man out of Ft. Worth. At this point, everything is new. Pwr brake booster kit from Tuckers specifically for disc/drum setup. New rear drums and shoes. New flex lines. The only things not brand new are the rear wheel cylinders (no leaks, look nearly new) and the return springs in the rear drum brakes (kit was backordered when everything else came in so went with the old ones). Did not replace hard lines, but blew them clear.
Before the conversion, the old girl didn't stop fast, but she didn't pull either.
Now, when I step on the brakes hard the truck pulls hard right. To me it feels like the back end is trying to come around. The right rear wheel locks up right away. It does not "feel" like the front brakes are pulling. To me it feels like something in the rear. I have adjusted the rears manually a half dozen times -- loose, what feels right, and a little tighter than I normally would on my other vehicles that need adjusting -- no change. Any ideas appreciated. Thanks