Abstract: Service/emergency brake actuator mechanism for use with the air brake system of a wheeled vehicle. A plurality of breather holes or openings are provided in the housing section of the emergency brake actuator chamber that houses the emergency actuator spring to maintain the chamber at atmospheric pressure. The openings are arranged such that regardless of the orientation of the brake actuator mounted in the wheeled vehicle, one of those openings will be located in the upper hemisphere of the chamber housing and another opening will be located in the lower hemisphere of the chamber housing. A flow resistant plug member, adapted to fit into any of the openings, is provided to seal whichever opening is oriented in the upper hemisphere of the chamber housing. This arrangement minimizes the amount of dust and dirt particles that enter the chamber and, thus, keeps the chamber free of contamination that could hinder actuator performance.