We typically turn big tires on our 4WDs and when it comes time to slow down or stop, it's good to have a braking system that can handle the rolling inertia of that wheel and tire mass. That action starts with the mechanical movement of your brake pedal that actuates the brake master cylinder mounted to the firewall. From here, hydraulic fluid is pushed out through the connecting fluid lines to your calipers and/or brake cylinders at each wheel.
An automotive braking system is a hydraulic system where fluid pressure is transmitted to each corner of the vehicle to apply brake pedal force. This force is tremendous, and mechanical leverage and the advantages of fluid pressure applied to surface area are used to multiply that advantage.
Under your dash is a brake pedal that swings from a pivot point. Two lever arm locations hang from this pivot point. The first is a short arm (or sometimes a connecting point higher on the brake pedal arm) that connects to a small steel rod that enters the master cylinder through the firewall. The second is the brake pedal where you place your foot. The brake pedal is much longer than the master cylinder lever arm. This is the first mechanical advantage in the brake system. The ratio of this mechanical multiplier may be about four times as we trade movement at the pedal for about a quarter of the movement and four times the force at the master cylinder rod. The goal is to build tremendous pressure at the brake cylinders to apply the brake pads without requiring too much legwork.
The rod then passes through the firewall and may enter the master cylinder unboosted, or may enter a vacuum or hydraulic power booster cylinder. The majority of vehicles use a vacuum-assist booster that is powered off the intake vacuum of the engine. Modern brake master cylinders contain two pistons to feed two separate fluid lines. This allows the four brakes to be separated into two sets of two for safety reasons. Should one fluid line ever fail, you are still left with two brakes intact.
Brake master cylinder and...
Brake master cylinder and vacuum booster
A vacuum booster mounted to the firewall works by using the energy of the engine intake vacuum (or a vacuum pump) to help reduce the braking effort your foot must apply. The booster consists of a large sheetmetal canister that is mounted to the firewall between the brake pedal rod and the brake master cylinder. The force of the input rod is exerted on a bladder that essentially has ambient pressure on the input side, but is under a vacuum on the backside. This differential pressure is used to assist the action of the brake rods. A valve within the booster prevents the pressure differential from developing until at least some pressure is placed on the brake pedal by the driver. This prevents any slight application or dragging of the brakes when they are not in use.
A hydraulic booster works in much the same way as a vacuum booster to help provide mechanical advantage to the master cylinder. It too bolts between the firewall and the master cylinder. In this case, the source of power is the power steering pump. It provides fluid pressure to the hydraulic booster that is used to help push the master cylinder rod.
With both vacuum and hydraulic boosters, the input rod from the brake pedal enters the booster and is intercepted and routed through the boost mechanism. However, in both cases, should the boost system fail, the pedal input rod will still contact the input rod at the master cylinder to operate the master correctly. This fully manual fail-safe mode allows the brakes to still operate unboosted, should the boost system fail.