Standard 121: Air Brake Systems
Scope and Purpose:
This standard specifies performance and equipment requirements for vehicles equipped with air brake systems, including air-over-hydraulic brake systems, to ensure safe braking performance under normal and emergency conditions.
Application:
Trucks, buses, and trailers equipped with air brake systems
Certain trailers and trucks are exempted depending on width, axle GVWR, maximum speed, and unloaded vehicle weight.

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