Master Cylinder Choices

[TurboTwin]

Hi All!

I've been a lurker for almost three years now, and while doing so has answered MANY of my q's, I haven't found an answer to this one challenge...

I am building a G-Body with a complete set of C5 brakes front and rear, with Astro Hydroboost. Right now I'm stumpd on Master Cylinder choice.

I've read much about bore size, and I just want to make sure I'm measuring the right area; anyone have a pic of exactly where to measure? I have several old masters sitting around the Shop, (Astro van, Caprice, G-body, 1st gen s-10) yet I need help I'd'ing which (if any), would be best for my application.

I intend to run an adjustable prop valve to the rear, off a *gutted* combo valve (prolly from the caprice, as it looks all shiny :D) since I'd like to keep my balance warning light.

I can purchase an LS1 master; my gut feeling is that it would be a good match for the swap, yet I like using what I've got kicking around (done so for most of this build) and I want to learn about the guts of masters and how they actually work. Also, I'm not sure the flange will fit into the Hydro.

So, basically, how exactly does one measure bore size? all these masters look very similar, just the reservoirs look different, and the mounting flanges dont all fit into the hydro unit. Will the Astro unit be enough master for this app? (to confuse me even more, two of the Astro Cyls have different reservoirs on the rear circuit; does this impact fluid volume to the calipers?)

Also, I've read about residual valves and how its not a good idea to have one for drums on a disc conversion, yet I can't really seem to I'D this part on any of these masters.

Sorry for the long post, just want to make sure I communicate clearly :P

Whew. Educate me at will!

Thanks!