When vehicle manufacturers design an OEM braking system, they match the size of the braking components to the weight of the vehicle, and then match the size of master cylinder to the wheel cylinders or calipers at each wheel. That works fine for a stock vehicle with stock tires, but that might not do for us. As we go to heavier tires, and even more weight as we swap axle components to handle the bigger meats and more rugged terrain, we’re often faced with upgrading or swapping brake components for larger ones.
Typically, we would swap to larger axles and with that end up with larger wheel cylinders or calipers. When the stock master cylinder is tasked with operating these bigger brakes, it must provide more fluid, resulting in a longer pedal stroke to get the rig to stop. The pedal will end up falling closer to the floor, or in extreme cases, a double-pedal pump may be required to stop the brakes hard.
The answer is often to upgrade the master cylinder to one having a larger bore. It’s often best to calculate the increase in size in your new brakes and match it with a like increase in master cylinder bore. One common mistake is to increase the master bore beyond what is needed. This results in a hard pedal and a vehicle that’s less fun to drive. Taking a bit of time to do a correct match will pay off in better braking. OR
The effective power of your master cylinder is dependent on the bore size. For a given push of the brake pedal, a smaller bore master will provide more hydraulic pressure out to the brake cylinders and calipers at the wheels. A larger bore delivers more fluid to the brakes, but at lower pressure. Hence, there’s no reason to increase the bore of your master cylinder if the one you have can provide braking without excessive pedal travel. A larger bore master in this case would decrease pedal travel, but will require more leg power to apply the brakes.
One final way to tuning the front-to-back bias of your new brakes is to add a fluid proportioning valve to the line running to the rear brakes, which require less braking force than the fronts. A valve such as this can reduce the immediate rear line pressure to lower the braking force at the rear.
Larger brake calipers require more fluid across their piston surface to get the same pad movement to the rotor compared to the action of a smaller caliper. Sliding calipers having multiple pistons are calculated using their combined surface area of the pistons. Fixed calipers that have pistons on both sides of the rotors are calculated using the surface area of the pistons on one side of the rotor. You have twice the number of pistons of a sliding caliper, but each must only move half as much to contact the rotor surface.