As of my last knowledge update in September 2021, there were several top-rated manufacturers of air brake systems in the USA. Please note that the rankings and information might have changed since then. Some of the prominent manufacturers at that time included:
Bendix Commercial Vehicle Systems: Bendix is a well-known manufacturer of air brake systems and related components for commercial vehicles. They offer a wide range of products including air compressors, valves, actuators, and electronic safety systems.
WABCO (now part of ZF Friedrichshafen): WABCO was a major player in the air brake systems industry, known for its innovative technologies and solutions. In 2020, ZF Friedrichshafen completed the acquisition of WABCO.
Haldex: Haldex is another reputable manufacturer of air brake systems, focusing on products like air disc brakes, brake adjusters, and air treatment systems.
Meritor: Meritor is a supplier of various automotive components including air brake systems, axles, drivelines, and suspension systems.
Velvac: While not as large as some of the others on this list, Velvac specializes in manufacturing air brake and air control systems for commercial vehicles.