Abstract: A control system for a combination hydrodynamic and friction retarder brake wherein in an output speed range above a first reference speed retarder braking is entirely hydrodynamic, in an output speed range between the first reference speed and a lower second reference speed retarder braking is partially hydrodynamic and partially friction with friction braking increasing as output speed decreases to a maximum at the second reference speed, and in an output speed range below the second reference speed both friction and hydrodynamic retarder braking decrease as output speed decreases to provide a smooth transition to the vehicle's service brakes. In addition, a partial apply valve is provided to limit retarder braking in all speed ranges to levels proportional to a manually applied retarder brake demand signal.