Front brake master cylinder

Several Yamaha dirt bikes used a 10mm Mc that is bolt on, even has the correct mirror mount. and brake light switch. SS brake line is needed with the smaller MC bores.
 
so the different bore sizes that are on these are those for the size of brake line.

Hi Jmann,
It's the ratio between the areas of the brake lever piston and the caliper piston that's important.
The stock brake lever piston area is perfect for working the two calipers the rest of the world's XS650s have.
Thus it's twice as big as it should be to work the single caliper they put on North American XS650s.
That's why you need a brake lever that has a smaller piston.
The brake line size has nothing to do with it except it best be a stainless line so it don't bulge under pressure.
Mike Mercury's Vintage Brake site has a chart to tell you all about piston area ratios.
 
Wow.. I feel like an idiot. I didn't think about the bore size when I swapped in a brake master cylinder from a kawasaki zzr1200. Looks like it is a 5/8" bore and when running with a stock 38mm caliper comes out to 5.73, feels terrible. Thanks for the post
 
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