such that they connect the respective line branch 68 a or 68 b to the tank in a non actuated position and interrupt a connection between the line branch 68 a and 68 b and the tank in an actuated position as in the control device 52 shown in fig2 control fluid can be displaced out of the controlled control pressure chamber of the directional valve 54 via the return vale 64 the line branch 68 a the check valve 78 and the pressure relief valve 74 to the tank 58 in the control device 72 in the event of a blockage of a pilot control valve u2014 in the following the pilot control valve 60 as an example control fluid can thus be displaced out of the left hand control pressure chamber via the check valve 64 until the control slider has returned to its neutral position by controlling the pilot control valve 61 and by the effect of the restoring springs on the control slider of the directional valve 54 in addition the line branches 68 a and 68 b can be relieved to the tank independently of one another by the switching valve 75 and 76 respectively in the normal operating state the switching valves 75 and 76 are actuated i e they interrupt the connection between the line branches 68 a and 68 b and the tank on a blockage of e g the pilot control valve 60 the switching valve 76 can be switched into the non actuated position so that the line branch 68 a is pressure relieved control fluid can then flow out of the left hand control pressure chamber of the directional valve 54 via the check valve 64 toward the tank the control slider of the.