Brakes are pretty much the most important

The master cylinder, the heart of the vehicle’s braking system, holds the brake fluid when it is not being delivered to the brakes through the brake lines. If brake fluid leaks because the master cylinder is worn or brake lines are plugged or broken, the fluid cannot be delivered, and the brake pads will become ruined.

The brake fluid itself can become dirty or contaminated as it draws rust-causing moisture and picks up other debris, or it can break down from excess heat. Clean brake fluid is either clear or slightly yellow, while dirty brake fluid may be brown or even black. Old and dirty brake fluid can damage ABS brake systems internally.

The brake lines connect to the master cylinder through a combination valve, which combines a metering and proportioning valve. It regulates the pressure on the front and rear wheels to make sure both sets of brakes are applied simultaneously. A malfunctioning combination valve may cause the wheels to lock up.

Brake pads and shoes can be made of ceramic, metal or organic materials, while the disc rotors and drums they press against are made of metal. Because the pads and shoes create friction to stop the car, they gradually wear down over time and may wear away completely, letting the metal of the calipers and cylinders they are attached to grind against the rotors and drums and damage them. Some pads have a metal strip attached that sounds a warning whistle when the pad becomes too worn, but this strip sounds only when the car is in motion and the brakes are not applied.