I'm betting that a 70 wheel cylinder is big. if you only have a 3/4 bore on the master and say a 3/4/ or 7/8 maybe even 1" or 1 1/8" on the rear or maybe even bigger will that make a difference? I know some people put a gm 1 ton cylinder on the back. I'm guessing you will need a bigger master to make that work. I have know idea what a gm master cyl bore is... maybe a good place to start and compare from.

also double check to make sure your shoes are not too far from the drum and you're not using up all your stroke on the MC before you even make contact w/ the drums