Whether it's a heavy vehicle like a bus or a truck, or something smaller like a car or motorcycle, vehicle braking systems are there for one thing, and that's to slow a vehicle down, and when it's stopped, keep it there!
Foundation brakes are there to convert the kinetic energy of vehicle movement to heat energy by friction of the brake lining material against the discs/drums. Basically when a vehicle is moving, it has kinetic energy. In order to stop the vehicle we need to get rid of that kinetic energy by converting it to heat energy.
Generally, on smaller vehicles like cars and motorcycles, hydraulic and even vacuum assisted / servo boosted brakes are used as the primary method for braking. Where on larger vehicles, like trucks and buses, you'll generally find air braking systems.
So, why is that?
Well, it's all about the amount of force that you need to apply to the brake linings to generate that heat. The larger the vehicle, the more kinetic energy it carries, so in turn the more heat energy you need to generate to slow the vehicle down. If you've ever driven a car, you'll know that when it's heavily loaded, you need to apply a lot more force to the brake pedal to slow it down. Now, as the vehicle's weight increases, the amount of braking force needed increases, and there is only so far we can go with our leg power and a hydraulic braking system. There is something that we can do to our heavier cars and that's fitting a vacuum boosted braking system (servo assisted), to give us a bit more force, but that's only going to get us so far. There comes a point where we need to used compressed air to supply the braking force for us.
Find out more about Air Braking Systems on the Air Brakes page.
What are brakes for? and how do they actually work?
Hydraulic brakes or Air brakes?
So, how do we get more braking effort from hydraulic brakes?
As vehicles get bigger, the amount of braking force we need gets greater, and there's only so much force we can apply with just our legs. One way that manufacturers originally overcame this was by using the vacuum created in the vehicles inlet manifold to assist the drivers foot effort, boosting the power applied to the master cylinder. This device goes by many different names, depending where you are in the world, the most common ones are power booster, brake booster and vacuum servo.
Over the years, as engine design has changed and with more and more fuel injected and turbocharged engines, the depression created in the inlet manifold is no longer sufficient to operate a vacuum assisted hydraulic braking system, so you are more likely to find vacuum pumps on modern vehicles.
Find out more about hydraulic brakes on the hydraulic brakes page.