In order to have one motion (the pressing of the brake pedal) into four motions for brakes, and at two different levels, a brake master cylinder is needed. The component uses hydraulic fluid to divide one movement into two or four, depending on the model.
Similar to the effect of leverage, if two tubes of different widths are connected by a pipe, and there is a cylinder in each tube, pushing the fluid out of the smaller tube would be easier, but the larger tube would fill up slowly. In the opposite way, pushing the fluid out of the large one would be more difficult, but it would fill the small tube up quickly. This is used to alter how much of the pressure is sent to the front brakes instead of the rear (this is known as brake bias).
Brake bias is used to describe the difference between the distribution of braking force between the front and rear brakes. It can also refer to left and right braking force differences, but on most passenger and racing vehicles, the left and right brakes always receive even force, so left-to-right brake bias is rarely encountered or discussed. While the brakes on the front of a vehicle tend to be larger to deal with the bulk of the braking effort, the master cylinder may still have bias built into it to apply even more pressure to the front brakes.
Getting Bias Balance Right
Striking a balance between the front and rear brakes is important on every vehicle, from economy vehicles to F1 race cars. Too much bias towards the front may take away grip used to steer the car, or lock up the front wheels, while too much bias towards the rear can cause rear stability issues, or lock up the rear wheels. Both almost always causes the driver to lose control of the vehicle.
On production vehicles, ideal brake bias is worked on in development, before the vehicle goes into full production. As shown in the above video, in some racing vehicles the brake bias can be adjusted by the driver mid-race.