A common question is ‘What bore size master cylinders should I use? In theory, it is possible to determine the correct size of the master cylinder (piston diameter) by calculating pedal ratio, pedal travel and caliper piston diameters but it’s often easier to ‘suck it and see’ by choosing a .750" master cylinder to begin with and working from there.
Many factors will determine the optimum master cylinder size such as weight of the vehicle, tyre diameter, brake pedal length, weight distribution and servo or nonservo assistance. Brake pedal ratio is often the easiest parameter to change. Increasing the pedal length is, in effect, increasing the leverage but the pay-off is less fluid movement for a given pedal travel. If the pedal is 12" long from the fulcrum to the foot pad and the cylinder pushrod is 3 inches from the fulcrum then the pedal has a 3 to 1 ratio. Increasing the ratio to say 4 to 1 will give more leverage but with longer pedal travel.
For a clutch the same rules apply to the release fork. A higher ratio will make the clutch action easier but there will be less travel of the release bearing. Determine the distance the release bearing has to travel to fully disengage the clutch (usually about 10mm) and work backwards from there.
A release fork with 2 to 1 ratio will require 20mm travel of the slave cylinder.
Brake hydraulics is really all about fluid movement so, to give you an idea of the effect of changing master cylinder sizes, here’s a chart of how much fluid each size master cylinder moves with each 1cm travel of the push rod. You can see that a 1" cylinder will move 2 ½ times more fluid than a .625" cylinder. But the pedal will require 2 ½ times more force to get the same braking performance. However, the pedal travel will be 2 ½ times less. Of course, if you want to increase braking power without changing the pedal or cylinder you could just add a servo.
CYLINDER PISTON DIAMETER FLUID MOVED FOR 1cm OF PISTON TRAVEL
0.625" ( ⅝") 1.98 cc
0.750" (¾") 2.85 cc
0.825" (⅞) 3.45 cc
1" 5.06 cc