What Does A Brake Master Cylinder Actually Do?

We should probably start by saying a brake master cylinder technically isn’t 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 doesn’t 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’s foot. In most cars the pedal motion directly pushes a rod (known, funnily enough, as a pushrod) into one end of a sealed master 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 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.

So it’s as simple as that. The brake pedal squeezes two pistons inside the master cylinder, which in turn push brake fluid down two separate lines to send equal pressure to all four wheels. The two springs behind the pistons push back when the pedal is released, effectively sucking the pads away from the brake discs. And now you know how a master cylinder works.