Hi Gary,
How would I know what would work, us "weekend tinkerers"are only concerned with making sure our crash helmets, Tanks, Boots and handbags match each other.
I’m joking of course.
Sorry: I tried but I just could not resist it 😊
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The ratios are calculated from the cross section areas of the pistons, not the diameters, because there is an exponential element in the calculation, a small change in the diameter will make a large difference in the ratio.
A 13mm diameter master cylinder will give you a 23:1 ratio, an imperial 1/2" diameter will give 25:1, 12mm diameter master cylinder will give 27:1.
I like to have between 22:1 and 26:1 for feel.
However this does not match the original long lever, a standard rear brake will be locking up all of the time with a small diameter master cylinder.
Even the standard 5/8" diameter master cylinder (15:1) locks up very easily with the very long standard lever.
Reducing the diameter of the master cylinder is only viable if you have already reduced the mechanical advantage of the lever, buy fitting rear sets or moving the footrests back.
If it was my bike I would look for a master cylinder of 13mm, these are relatively common, this will add some power to the rear brake and better ‘feel’ (control) over the standard setup.
Then I would experiment by a mechanical lever system that can be adjusted, or quickly adapted. I would then try to build an input lever length to operate the master cylinder that allows me to lock the rear wheel, but I would have to work hard at it. If the wheel locked too easily I would reduce the mechanical ratio, if I could not lock the wheel at all then I would increase the mechanical ratio.
You should end up after a few experiments with a brake that is reasonably powerful, but is easy to control, giving good feedback to your foot.
Good Luck
Post some photos when you are done.
Regards
Peg.