An electronic vehicle braking system has a back-up braking system in which a brake pressure control valve (22) is actuated whenever the vehicle brake pedal is depressed. The output from the valve (22) is connected to a selector valve (24) which isolates the control valve (22) from the vehicle brakes when the electronic braking system functions correctly, but which allows the braking pressure from the control valve (22) to control the vehicle braking when the electronic braking system is faulty. Also disclosed is a particular example of control valve (22) which is suitable for use in controlling a spring brake (18, 20) in a back-up mode.