Had exactly the same issue when I switched out my M2 rotors for larger Granada rotors & GM metric calipers. Could not get any back pressure whatsoever ! It turned out to be a combination of issues which separately likely wouldn't have caused a problem, but together screwed things up. First, make sure your rear shoes are adjusted up properly. I changed Master Cylinder to a larger bore ( F 250 MC ) as GM calipers seem to need a little more volume. I also tweaked the booster piston a bit, although I had same issue with brakes seizing with too much adjustment. Also check the connection point between your brake pedal arm and the booster rod for too much slack.
Good luck !
