The average drivers don’t think about their brakes as often as they probably should. Almost everyone knows that brakes make a vehicle stop, though there are a few speed demons on the roads who act like they don’t know that brake pedals exist. Some drivers even know what a parking brake does, even if some of them leave it on when they drive down the road for the first mile from home. Regardless, truck brakes are a bit more complex. Essentially there are three different kinds of brakes that you’ll encounter in large trucks: service brakes (air brakes), parking brakes (spring brakes), and the emergency braking system.
For service brakes, also called air brakes, pressurized air is used to activate the system. These are by far the most commonly used brakes during your daily travel; however, some trucks use wedge brakes or disc brakes, which are far less common. S-cam brakes are the most commonly used type of service brakes in semi trucks. They get this name, appropriately, from their noticable S-shape. Semi trucks with air brakes that were manufactured after 1998 also are equipped with ABS, which prevents the wheels from locking up when a driver brakes hard.
Not to be confused with Spring Break, which is far more enjoyable, the spring brakes are primarily used to secure a semi when it’s parked. They do, however, have a more immediate purpose as well.
Spring brakes can also be activated as a result of air pressure loss, such as in an air leak or other emergency situation. They can be controlled through a knob in the cab. Typically, air pressure holds these brakes open unless air pressure can no longer overcome the pressure that the springs exert. Sometimes, a driver can disengage these brakes by utilizing a separate air tank so that the truck can be moved out of a dangerous situation. The driver can then slowly bring the truck to a complete stop.