Editing Brake (section)

===Brake boost===
[[File:2008-05-05 1990 Geo Storm GSi vacuum servo.jpg|thumb|right|Brake booster from a [[Geo Storm]].]]
Most modern passenger vehicles, and light vans, use a [[Vacuum servo|vacuum assisted brake]] system that greatly increases the force applied to the vehicle's brakes by its operator.<ref>{{cite web|last=Nice |first=Karim |url=http://www.howstuffworks.com/auto-parts/brakes/brake-types/power-brake.htm |title=How Power Brakes Work |publisher=Howstuffworks.com |date=2000-08-22 |access-date=2011-03-12}}</ref> This additional force is supplied by the [[manifold vacuum]] generated by air flow being obstructed by the throttle on a running engine.  This force is greatly reduced when the engine is running at fully open throttle, as the difference between ambient air pressure and manifold (absolute) air pressure is reduced, and therefore available vacuum is diminished. However, brakes are rarely applied at full throttle; the driver takes the right foot off the gas pedal and moves it to the brake pedal - unless [[left-foot braking]] is used.

Because of low vacuum at high RPM, reports of [[Sudden unintended acceleration|unintended acceleration]] are often accompanied by complaints of failed or weakened brakes, as the high-revving engine, having an open throttle, is unable to provide enough vacuum to power the brake booster. This problem is exacerbated in vehicles equipped with automatic transmissions as the vehicle will automatically downshift upon application of the brakes, thereby increasing the torque delivered to the driven-wheels in contact with the road surface.

Heavier road vehicles, as well as trains, usually boost brake power with [[compressed air]], supplied by one or more compressors.