The master bracket that I make will accept a GM master that you can get in a variety of different bore sizes! I tried out a few on mine and ended up with an 1 7/16" I believe. I started with an 1 1/4" and tried an 1 1/2" also. The bolt pattern is the same on all of the GM master that accept a vacuum booster. When you get into the hydra-vac I think is were the bolt pattern changes!
