Mushy brakes 4 life...

It seems that it's an issue of the rear brake cylinders not getting the volume of fluid that it needs. Are you sure that the rear section of the master cylinder is working right? I'm sure you bench bled it. I recall a time or 2 like that with a reman MC. Swap it out and fix. Make sure though that your shoes / drums are matched.

Now if you were to say that you had a right stuff disc brake kit I would tell you to make sure the bleeders on the rear calipers are right at the very top. I've seen and experienced that, had to bleed those with the calipers partially installed so that the bleeders were at the top in order to get all the air out, then bolt them into place.