Brake & Suspension Repair
Air brake systems are used on most heavy-duty vehicles for reasons of efficiency and reliability. The major advantage to an air brake system is that since air never runs out, the air brake system can always be replenished. An air brake system is also functional even if it has a small leak.
Heavy trucks use a Dual Air Brake System, which consists of two separate air brake systems that use a single set of brake controls. It is designed to retain braking ability in the event one system fails. The various components in an air brake system work together to create and maintain a supply of compressed air, direct and control the flow of that air and to transform air pressure energy into mechanical force.
Heavy trucks use air brakes exclusively. Most of these are drum type units. Air enters the chamber when the brakes are applied, the push rod moves out turning the slack adjuster which rotates the "S" cam and forces the shoes into the drum. Air brake systems use high pressure air - about 100 psi - to apply the brakes. Air is supplied by an engine driven air compressor and stored in tanks on the tractor and trailer. When the brakes are applied the air comes from the tanks, they are then recharged by the compressor.
Big trucks don't stop very well, when fully loaded they take about twice as long to stop as passenger cars. Most air brake systems do not automatically adjust for wear, so unless they are well maintained brake performance can deteriorate seriously. Roadside truck inspections frequently reveal that a fairly high percentage of the inspected vehicles have defective braking systems, generally because of air leaks.