there is illustrated a schematic representation of a relay valve with means engaging the relay piston for the initiation and control of the second phase of operation the components connections and chamber having the same function are again identified by the same reference numbers as in fig1 as shown a control chamber 4 is located above the relay piston 70 which is connected with the control connection 5 and a reaction chamber 11 is located below the relay piston 70 which is connected with the working connection located below the relay piston 70 is the double valve 6 7 9 in this embodiment the relay valve 70 is designed as a differential piston with a large upper control surface 71 and a smaller lower reaction surface 75 the differential surface 72 which accounts for the difference in surface areas between these surfaces is also situated on the bottom of relay valve 70 the reaction surface 75 of the relay piston 70 borders the reaction chamber 11 between the differential surface 72 and the housing 73 is defined a differential chamber 81 which is in communication with the second control connection 17 via a housing duct 80 and a pressure relief valve 77 78 the pressure relief valve 77 78 includes a valve element 78 which is.