Be sure the shoes are in the right location, longer one to the back shorter on the front. Get them swapped, and it will grab every time.
Also be sure you plumbed the master cylinder right. The first one I used was backwards from what I through. I had the rear brakes plumbed to the front portion of the cylinder and the front to the rear, it made for all kinds of funny stopping. The front brakes apply first, and having them plumbed wrong, made the rears lock right up with little to no front brake.
I had to add a adjustable proportioning valve and limit the fluid going to the rear shoes. Normal driving was OK, under a hard stop it would just lock the rears. Now with some limitation to the rear, its a stiffer pedal, but I live with it knowing under a panic stop its not going to lock and slide.
I used a disk setup on the front, and stock '79 GM rear drums, non-power master cylinder mounted under the truck with out residual valves. The adjustable proportioning valve came from Speedway Motors
Joe
