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" been running a 1972-1981 3/4" bore master cylinder on a 1987 FLST single disc for about 4 years or so. It works fine; yes, the effort required is more than the stock 5/8" bore, but not enough more to worry about."
Well, that's good news. I assume your original master cyclinder had the 5/8" bore with 12mm banjo? Did you replace the brake line/banjo fitting, or use an adaptor bolt? There is this piece available
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Banjo adapter bolt, 10mm threads using 12mm line. Use on our part No. 90-344 handlebar master cylinder. Comes with two aluminum gaskets. Chrome plated. U.SA MADE.
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This old master cylinder doesn't use banjo bolts. But I replaced the entire brake line as I was installing a hydraulic brake switch. I have an internal throttle, and no wiring/switches on the bars.
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