We should probably start by saying a Brake Master Cylinder technically is not essential. You could use cable-actuated brakes, if you liked, like the cheap items you get on entry-level mountain bikes. On the other hand, to stand the strain of stopping one or two tonnes of metal, plastic and humans over 10-20 years, the cable would have to be massive.
There are more practical solutions, chief among which are hydraulics. The fact that liquid does not compress makes it perfect for transferring force from one part of a system to another. When it comes to the brakes in your car, the master cylinder is the key component in making that happen.
Picture a brake pedal in your mind. Delve into the relative darkness of that imaginary footwell and push that pedal with your mind foot. In most cars the pedal motion directly pushes a rod (known, funnily enough, as a pushrod) into one end of a sealed Brake Wheel Cylinder filled with fluid, two pistons to displace the fluid, and springs to push back against the main force and return the brake pedal to its resting position when released.
Importantly, there are two exits from this tube. One leads to two diagonally-opposed wheels, while the other leads to the others. The two-line layout is a safety feature that ensures that even if a line should leak, you can still stop – albeit more slowly, with one wheel on each side and each end doing the job.
Let’s assume normal operation. Pressing the brake pedal pushes the two pistons, each sprung separately in a linear piston-spring-piston-spring layout, into the fluid, pushing the liquid down the lines into what are called slave cylinders; usually situated on brake calipers themselves. The slave cylinders then push the friction material onto the rotor.
Above the Clutch Master Cylinder, which is normally horizontal, is a vertical fluid reservoir. Its job is to make sure no (compressible) air gets into the system by retaining enough spare fluid volume to keep the system fully fed at all times and in all phases of its operation.
When the pedal is pressed, the piston in the Clutch Wheel Cylinder is pushed in forcing hydraulic fluid from the master cylinder through the tube and hose to the slave cylinder (on the left). The fluid pushes the slave cylinder piston toward the open end of the cylinder pushing the pushrod that connects to the lever on the clutch-operating shaft that in turn releases the clutch.
