An exhaust brake valve assembly structurally arranged to provide reliable operation with relatively quick response. The assembly includes a sliding gate valve member driven by a power actuator through a lost motion connection which generates a desirable impact force on the gate to overcome static frictional forces. The lost motion connection cooperates with a relief port in the gate to relieve peak pressure forces on the gate just prior to opening movement to additionally improve opening reliability and response. Still further, the actuator includes a spring return arrangement in which the force rate adjacent the closed gate position is substantially greater than the force rate at the open position so that adequate reserve return force is developed for opening while only a minimal spring force resists piston movement in the initial opening movement. A quick release valve is provided on the actuator to rapidly exhaust fluid from the working chamber of the actuator to develop a high level of kinetic energy during the lost motion period. The actuator is protected from extreme temperature by a heat barrier plate interposed between the valve housing and actuator. A piston rod of the actuator is sealed by a radially floating seal and includes a reduced diameter portion and opposed shoulders for simple connection with the valve gate.