OK, I think I got this figured out now. Bore determines pressure in the brake lines. Larger bore/less pressure, smaller bore/More pressure. Master Cylinder Piston volume determines volume available to transfer to the Caliper piston.
I took the Master I recieved apart to look at the pistons. The front piston (for the rear brakes) was about half the volume of the rear piston (for the front brakes). That indicates I only have half the volume available to transfer to the rear brakes comparied to the front. This is OK because I'm using single piston calipers in the rear and dual piston calipers in the front, so I will only be using approximetly half the volume when braking is applied to the rear brakes compared to the front but the pressure will always remain the same. Had I been using dual piston calipers front and rear I probably would have a problem with this Master Cylinder because I would need equal volume available front and rear. There also is no residual valve in the Master Cylinder. Apparently that was something that was on OEM Masters and replacement masters don't usually have them and you must install a residual valve in the lines between the Master and the Caliper if you are using rear drum brakes.
Problems arise with rear disk conversions when a Disc/Drum Master is used that doesn't supply enought volume to the rear brakes. Disks use more volume than drum brakes so the Master Cylinder piston may not provide enough volume from proper rear wheel braking. Drum brakes typically have a very small piston in the brakes and the residual valve keeps pressure in the lines at all times so they only require small amount of volume from the Master Cylinder comparied to the larger rear disk brake caliper pistons.
This link hand great information:
http://www.mpbrakes.com/MASTERS.htm
Don,
Can you tell me if the Vette Master has equal size pistons in it or if it has one smaller and one larger pistons?