This SAE Recommended Practice establishes performance guidelines for the capacity of the air reservoir systems used on trucks, towing trucks, truck-tractors, trailers, and converter dollies with GVWRs over 10 000 lb, designed to be used on the highway. NOTE: Compliance with this document does not guarantee compliance with the air reservoir requirements of FMVSS 121. The purpose of this document is to establish a minimum air storage capacity for highway-type trucks, buses, trailers, and converted dollies which use compressed air to actuate or transmit actuation power to the foundation brakes. A vehicle's air reservoir capacity is quantified by its ability to provide a brake application pressure which is adequate to stop the vehicle within the emergency brake stopping distances specified in Table II of FMVSS 121, after seven full brake applications with the air compressor disabled.