Big trucks use air brakes, smaller vehicles use hydraulic brakes. Air brake systems use high pressure air - about 100 psi - to apply the brakes. (The hiss you here from these vehicles occurs when the brakes are released.) 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. This has to due with current technology, it has nothing to due with the underlying physical processes. Some of these technological limitations involve the air brakes themselves, axles and tires are also a limiting factor however. The designed in limitations of the basic braking system may be actionable, this is a complicated question.
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 or poor adjustments. Leaks are a service or maintenance problem. Poor adjustment can be a service problem or a reflection of the manufacturer's failure to provide the available automatic slack adjusters.
For more information go to the Air Brake Tech Page
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