If you have a true four piston caliper then you need to muiltply the area of one piston by 4 not 2 which would make the area of the pistons 9.61625. The area of the master cylinder being small compared to the caliper area is what gives you some hydraulic advantage. Given the same pedal force and caliper a smaller master cylinder will create higher line pressure, but will require more pedal travel to actually clamp the pistons on the rotor. This is caused by the smaller diameter MC discplacing less fluid for a fixed pedal travel when compared to a lager MC. The higher line pressure will lead to more clamping force at the rotor given the same piston area. So there is a trade off between haveing a large diameter MC that you have to push really hard with very short travel or a small diameter MC that you have to push a long way but with less force. There are calculators online that can help with the MC sizing based on car weight, speed, piston area, and pedal ratio.
If I remember correctly brake boosters multiply the force that you input on the pedal, applying that greater force to the master cylinder. This makes the driver have to input less pedal effort for the same stopping power. If you have never driven a manual brake car they take a little getting used to.
Hope this is all correct been awhile since I sized the brakes for the schools FSAE car. Hope that helps
Edited by 383chevelle, 14 January 2015 - 03:19 PM.