What master are you using?
Starting in about 83 GM started to change the angles cut in the calipers for the square cut o-ring. This allowed the pistons to retract more from the rotor yielding more MPG because of less drag. However to do this they needed to use a step bore master cylinder. They had a large (40mm mostly) piston that would move lots of fluid and would transition to the smaller bore (1.125" in the case of the caprice) to keep peddle effort down. As you found out mixing the 2 up doesn't work too well. I also bet that you have way too much braking in the rear. Best bet would be new MC with a larger bore and smaller calipers in the rear (like metric calipers)