Why is a fluid used in brake fluid?

Answer 1

We know that "fluid" used as brake fluid needs to be almost incompressible to be effective. The fluid used in brake fluid is a liquid, and when you step on the brake, this liquid is put under pressure to transfer the force to the pistons in the brake calipers (on systems with calipers and rotors) or brake drum pistons (on systems with brake drums and shoes).

If we used air, which is a gas, the air would compress and warm up, and little "power" would be transferred to the working components of the system. We couldn't stop. But liquids (like brake fluid) are largely incompressible, and they will very effectively transfer the energy of the foot (through the master cylinder) to the pistons in the brake system on each wheel.

As a closing note, both gases and liquids are considered fluids by physics types. They study behavior of fluids in what is called fluid mechanics. But as for hydraulics, which the industry has dubbed "fluid power" as a distinguishing umbrella tag, we see hydraulics in action. A trash truck used fluid power to lift and dump the dumpsters on its route. A lot of the little skid loaders or skid steerers (like Bobcat and others) use fluid power to drive the wheels and operate the bucket, blade or whatever accessory is bolted on and hooked to the hydraulic system.