The correct bore size of the front or rear brake master cylinder is dictated by the number of calipers used and size and number of pistons on each calipers. On stock motorcycles the rule of thumb is: 5/8" bore for single caliper early F & R and most 77-83 dual disc front 9/16" bore for later front single caliper 84 up 11/16" bore for later dual disc dressers like FLHR 3/4" bore for early dual disc dressers like 80-83 FLT It's best to use the recommended one for your motorcycles year/model, but if you change to multi-piston calipers like Performance machine, GMA, or Jaybrake, you can increase or decrease bore size to suit your taste. Decreasing bore size increases lever travel and stopping power. Increasing bore size decreases lever travel and gives a much firmer feel to brakes. There are limits though and using too small a bore size for multi-caliper/multi-piston braking may not provide enough volume to stop adequately. Pre-2000 HD brakes are known for being somewhat marginal, especially for heavier models and when towing a trailer. A popular upgrade is to replace the stock calipers with aftermarket ones that have multiple pistons that clamp from both sides of the rotor and have larger pads for more swept area. The leader in high performance brakes is Performance Machine and they also make rotors and master cylinders for performance applications.