Master Cylinder Bore?

Couple of things to consider.
- fluid does not compress.
- smaller diam m/c gives more pressure & therefore less pressure reqd on the pedal [ compared to larger diam m/c ]
- not all m/cs are created equal. Bigger discs often have larger caliper pistons. While the 15/16" m/c might be the 'right' size for pedal feel & pressure, it displaces less volume. The m/c needs enough volume for the job, & bigger pistons need more volume. So volume requirement needs to be checked out.