Why is a liquid used in a car's braking system?

Answer 1

A liquid is used because it cannot be compressed.

Remember the scientific qualities of the three states of matter. Solids have a definite shape and volume. Liquids have a definite volume, but not a definite shape. Gases can not only take the shape of a container like liquids, but with compression, a wide variety of different volumes of gas can fit into the same sized container.

Now, you want a 1:1 relationship with the medium in the brake lines and for it not to compress. The reason you bleed the brakes is to get the air out so the brakes won't be spongy, since any air in the lines can be compressed. As hard as the brakes are to operate, a significant amount of air would mean that nothing would happen when you press the brakes. The air would absorb the pressure and the fluid past the air would never get the necessary amount of pressure to operate the brakes.

Now trucks do use air brakes, but the system is too bulky to be practical in cars, and not as responsive. They employ a number of workarounds and tricks to make the system work. For instance, most brakes in tractor-trailers require air to release, not apply. That way, they can be much more responsive, since they are operated by heavy springs and the air fights the pressure of the springs. So the brakes are not always prompt in releasing. For the brakes to work in the trailer, there is a relay system, where a separate air tank under the trailer is kept charged, and thus the lines from the truck signal when to use the air from that tank. That is because the distance from the auxiliary tank under the trailer is much closer to the trailer air brakes than to the air tank in the truck.