Abstract: The invention relates to a spring brake assembly of the compressed air type for truck tractor types of vehicles. The brake actuators of the assembly each includes a heavy coil type emergency spring which is held in a compressed state on the back side of a diaphragm by compressed air applied to the front side of the diaphragm during operation of the vehicle. The emergency spring expands to actuate a brake setting push rod when the compressed air is vented from the front side of the diaphragm and this occurs when the vehicle engine is turned off or when there is an accidental loss of compressed air from the brake unit. The emergency spring is housed in a chamber and, due to the movement of the diaphragm, atmospheric air is continually being drawn into and expelled from that chamber. As the brake actuators are mounted on the rear axle of the vehicle, they are in low positions relative to the roadway and the air drawn into the emergency spring chambers thereof is laddened with salt and water.