I used a manual master cylinder from an early 80s truck. It's WAY too big for the M-body calipers in front and Caddy rear calipers.
Hard braking usually required firm application of both size 15 feets and 225 lbs of my chunky butt hovering above the seat.

I put the monster AMG calipers on the front and now the pedal feels just right.

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon