So I finally converted the old nag from 1.5" to 2.25" wide rear brakes last year. I used the wheel cylinders that were on it already, they were in pretty good shape. If you check the specs for the mythical 6 cyl convertible rear brakes, there's a slightly different diameter cylinder listed there, though not in parts catalogs. I've also noticed that for the parts places that list 6 cyl rear wheel cylinder diameters in their description, they often disagree with one another.There is a small change in spec from early 65 to later, maybe that gets them confused. Next time I buy any I'm gonna measure the bore and see if it's as advertised. Anyway, there are a few slightly different diameter cylinders out there. I didn't drive it much after the conversion because I'm usually working on something these days. I didn't even insure it until the end of summer last year. So the brakes aren't broke in enough to make any serious judgments yet. I didn't notice anything other than they worked fine when I did drive them. But someday I'll be able to slide around a little and see if there's a problem with the front/rear brake bias. Yes, I could put in a proportioning valve, but couldn't I achieve the same thing tweeking the rear cylinder diameter? Please save me from finding my old physics book and looking up hydraulics formulas. Although with the wheel cylinders and the master cylinder (Look out Felix!) having similar diameters, that shouldn't be a huge factor. It seems because the brake fluid displaced by the master cylinder has less volume to fill in a smaller wheel cylinder, it would want to move out farther. Seems like that would make it brake harder. Or maybe the hydraulic laws say different, and bigger is better. I've been impressed with the depth of technical expertise some of the people on here have shown. I appreciatively await your wisdom.