I had a similar problem when I installed the TSM rear discs on the hot rod. That install really gave me fits. I tried multiple master cylinders and a small tanker ship worth of fluid.

I took the lid off the master cylinder, pulled the rear calipers off, and opened the bleeders. While gravity bleeding I held each caliper in my hand to make sure the bleeder was absolutely the highest point of fluid contact. The way the calipers were cast it was a bit misleading.

Bleeding them like that solved the problem.