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.

Abstract: A brake unit having a hydrodynamic retarder providing brake torque proportional to speed and inlet pressure, and a friction brake providing brake torque proportional to net brake pressure, the brake apply pressure minus retarder inlet pressure. The controls have a brake demand valve providing a brake demand pressure. A retarder valve is biased by brake demand pressure from brake-off position to brake-on position for switching the retarder circuit to fill the retarder and to connect low and high speed regulator valves to regulate retarder inlet pressure respectively to increase in the low speed range and to decrease in the high speed range with increasing speed, and to limit retarder inlet pressure proportionally to brake demand. The controls, in the low speed range, provide friction brake apply pressure proportional to brake demand, so the friction brake net apply pressure and torque decrease to zero and retarder torque increases as speed increases.