A control system for pressurised medium-operated friction brakes combined with a hydrodynamic retarder of a motor vehicle comprises a first pressurised medium source 1 connected by ducts 7;7c to a hydrodynamic retarder 2 which has a pressure medium actuated control member 11. A second and a third pressurised medium sources 3 and 5 supply the medium via respective ducts and control valves 12a; 14; 12b and 13a; 15; 13b to friction brake assemblies 4 and 6 of the vehicle. The duct 7 divides into two branches 7a and 7b which respectively contain control valves 9a and 8. The valve 9a is operated by a pedal 16 and controls the flow of medium to the retarder 2 during short-term braking. The manual valve 8 controls the flow of medium to the retarder 2 during long-term braking. The two branch ducts 7a and 7b re-unite in an automatic shuttle valve 10 which has two alternatively blockable inlets. The shuttle valve 10 is so constructed that the inlet carrying the medium at higher pressure is connected to the outlet 7c whilst the other inlet carrying the medium at lower pressure is blocked against return flow. The pedal 16 is arranged to operate successively the valve 9a, for controlling the retarder 2, and the valves 14; 15 for controlling the friction brakes assemblies 4;6. IMAGE
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