[Scarebird]

Originally Posted by scott06

the volume of brake fluid to move a caliper piston vs a wheel cylinder is what drives the change in Master Cylinder bore diameter

Not true.

A caliper piston while having a much larger surface area only moves a small amount. A drum cylinder moves quite a bit. so the actual volume is almost the same - at least for GM's of our era...

The reason a disc master has such a large reservoir is as the caliper pad wears that much more fluid is pulled into the caliper to make up for the piston travel.