This SAE Recommended Practice provides instructions and test procedures for air braked vehicles including but not limited to trucks, truck-tractors, trailers, dollies, and buses used on highways but does not include off-highway vehicles. This document provides a method of determining the air reservoir capacity of highway type vehicles using compressed air to actuate or transmit actuation power to the foundation brakes. Air reservoir capacity is the ability to provide adequate brake pressure after a number of brake applications.