A control system of a pneumatically suspended raisable and lowerable trailing axle (not shown) of a commercial vehicle with a steel-sprung driving axle 3 comprises a pressure-limiting valve 23, a control valves, a one-way restrictor 7 and a pressure-regulating valve 5 associated with the driving axle and a control valve 15 and a levelling valve 10 associated with the trailing axle. In this system, a pressure maintained constant by the pressure-limiting valve 23 is applied to the pressure-regulating valve 5, which determines a pressure proportional to movement of the driving axle 3 and which, after a pre-determined pressure level corresponding to an axle loading of approximately 95% of the safe driving-axle loading has been exceeded, causes a control valve 8 to be reversed, whereby a valve 12 vents trailing axle actuator 18 and air pressure is transmitted to the levelling valve 10 and, depending on the loading, pressure air is directed through the latter valve to the pneumatic suspension means 1, 2 until a predetermined distance, dependent on the loading, is established between the vehicle frame and the trailing axle. IMAGE
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