Tractor brakes are air brakes, whereas automotive brakes are hydraulic. With hydraulic brake systems, a brake pedal is pushed, releasing fluid to expand a hose that pushes against a brake pad, causing the rotor to slow or stop. With air brakes, valves filled with air hold or release brakes via the pressure of the air.
A hydraulic brake has a reservoir that contains fluid and a master cylinder that pushes fluid from the reservoir to the hoses. The brake hoses are made from rubber or steel and run from the cylinder to each wheel's caliper. The caliper contains a piston, the brake pads and a round rod that expands when the fluid reaches it, and it is mounted on a static place in the car. A brake pad consists of a metal pad that grips onto the rotor and a steel disc on the wheel that slows or stops it.
An air-brake system uses a compressor that pumps air for storage and a governor to control the amount of air taken in or released. A reservoir tank stores the air, while the drain valves relieve the pressure in the tank when the vehicle is not on. A foot valve releases the air from the tanks. The brake chambers contain an adjuster with a diaphragm, while a push rod connects the chamber to a slack adjuster and releases the brakes if pushed or applies the brakes if extended. The slack adjusters marry the push rod to the s-cam, which moves the brake shoes against the drum, to settle the distance between the brake shoes. Brake shoes are made of steel and create friction against the brake drum, while the return spring moves the shoes back to an open position when they are not in use.