FAQs for Semi Truck Brakes
Semi trucks have complex braking systems. There are service brakes, which are operated by pressure placed on the brake pedal either through air pressure or by hydraulic pressure; parking brakes, which must be used only during parking and are operated by a series of heavy-duty springs; and the emergency brake system, which is simply a combination of parts from both the service brake and parking brake systems. Within each system there are variances as well.
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What are Air Brakes?
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The air brake system is a braking system that uses pressurized air to activate the braking mechanism. It comprises the brakes used during normal driving. Pressurized air pushes the brake shoes and linings against the drums or discs to control vehicle speed when the foot brake is depressed. The most common type of brakes used in semi trucks are S-cam brakes, so called because the mechanism used to push the shoes against the drums is shaped like an S. There are also wedge brakes and disc brakes, but these are much less common. Semi trucks with air brakes, manufactured after March 1, 1998, are also equipped with ABS, which keeps the wheels from locking up during hard braking.
What are Spring Brakes?
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The parking brake system comprises brakes that are activated by a loss of air pressure. This air pressure loss can occur through an air leak or other emergency or can be controlled manually through the push/pull knob in the cab. Air pressure holds these brakes open during normal driving conditions. They activate when the air pressure can no longer overcome the pressure being exerted by the springs. The spring brakes are used primarily for securing the semi when parked.
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What Makes up the Emergency Brake System?
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The emergency braking system activates the spring brakes when the vehicle loses air pressure in an air brake system. The spring brakes engage engage when the air pressure drops below 20 to 45 psi in the air storage tanks, thus stopping the vehicle. These brakes can sometimes be disengaged by the use of air located in a separate tank (if so equipped) in order to move the vehicle out of a potentially dangerous situation. These brakes can also be applied manually in the event of brake failure in vehicles equipped with a modulating valve. The modulating valve allows the driver to gradually apply the parking or spring brakes through the use of a lever in the cab in an effort to slow the truck down. When parking a semi truck with a modulating valve, the level must be placed in the "fully on" position and locked in place with a locking device.
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