Let's begin by considering the ways that truck and car brakes are alike. The purpose of brakes on all types of vehicles is to stop them. Brakes on both trucks and cars work on the principle of friction. Both kinds of vehicles have brake drums, along with their pads and shoes, connected to the vehicles' wheel axles.
Car brakes rely on the brake fluid that flows through the system to work properly. Thus, car brakes are a hydraulic system, relying on fluid. On the other hand, truck brakes depend on compressed air. (Trains and buses also use this type of brake system.)
A major plus to using air is that it never runs out (as brake fluid can). This means that the air brake system is very reliable -- even if there's a small leak somewhere in the system, it always works.
Most of the newer heavy trucks use a dual air brake system that is not available on automobiles. A single set of brake controls works both of these separate air brake systems. If one system fails, the other will work.
The flaw in the compressed air system of trucks is brake lag. That's the time it takes for air to get through the lines and force the linings to contact the drum. When they push the brake pedal, drivers must get used to the fact that air brakes don't work at once, as they do on a car. Lag time is less than a second, so this is not a major problem.
The truck's air braking system has several tasks. First, it keeps up a steady supply of compressed air. In addition, it must direct that air's flow. Finally, it uses the energy of air pressure and changes it into mechanical force.
One, two, three! A truck's air brake system is really made up of three different brake systems. Let's investigate each of them.