fig1 shows diagrammatically a 4 / 3 - way valve 1 , thus a valve with four connections and three positions , which serves to control a hydraulic piston motor 2 which is to raise or lower a load 3 . for this purpose , the motor 2 has two control lines a and b which open into working chambers 4 , 5 of the motor 2 . the four - way valve is here arranged in a known manner between a pump 6 which serves as pressure source and a tank 7 which serves as pressure sink . downstream of the pump 6 are connected a pressure regulator 8 and a flow limiting means 9 , as well as a non - return valve 10 opening in the direction of the four - way valve 1 . the four - way valve 1 has connections p ( to the pump ), t ( to the tank ) and a , b ( to the motor 2 ). the 4 / 3 - way valve is built up of four two - way valves 11 , 12 , 13 and 14 , two of which communicate with the pump 6 , and therefore are designated as pump valves 11 , 12 , and the other two communicate with the tank 7 , and are therefore designated as tank valves 13 , 14 . the 4 / 3 - way valve is built up in the manner of a bridge circuit , in which the two - way valves 11 to 14 are arranged in the arms of the bridge , while one diagonal lies between the pump 6 and the tank 7 , and the other diagonal lies between the working connections a , b of the motor 2 . accordingly , the individual valves are always operated in pairs , by means of control valves 15 , 16 , which are in the form of electromagnetic valves and are arranged in a control housing 41 . the control valve 15 controls the actuation of the valves 11 and 14 , while the control valve 16 controls the actuation of the valves 12 and 13 . the construction of the valves 11 to 14 is revealed by fig2 . in a valve housing 17 , which is connected to the control housing 41 , there is provided for each two - way valve 11 to 14 a bushing 18 , 19 which is formed , for example , of brass or of a material from the group of high - strength thermoplastic plastics materials based on polyaryletherketones , especially polyetheretherketones , polyamides , polyacetals , polyarylether , polyethyleneterephthalates , polyphenylenesulphides , polysulphones , polyethersulphones , polyetherimides , polyamidimide , polyacrylates , phenolic resins , such as novolak resins , or the like , wherein glass , graphite , polytetrafluoroethylene or carbon , especially in fibre form , can be used as fillers . further , each valve 11 to 14 has a closure element 20 , 21 having a bearing portion of stainless steel . the material pairing between the bushing 18 , 19 and the closure element 20 , 21 is in any case so selected that the two parts can slide with low friction upon each other . since the valve is to be operated with water as the hydraulic fluid , a further pre - requisite is that both materials are corrosion - resistant . each closure element is subjected by a spring 22 , 23 to a closing force which presses the closure element 20 , 21 in the direction of a valve seat 24 , 25 . in order to improve the leak - tightness , the closure element 20 , 21 is provided at the end facing towards the valve seat 24 , 25 with an insert 26 , 27 of plastics material . while the valve seats 24 , 25 may likewise be formed of stainless steel and therefore have a certain hardness , the plastics material of the inserts 26 , 27 is softer , so that the valve seat can press itself into the insert and therefore results in a tight seal . the inserts 26 , 27 differ between the pump valves 11 , 12 and the tank valves 12 , 14 . the insert 27 of the tank valves 13 , 14 has a simple channel 28 which communicates with the corresponding working connection a , b of the motor 2 . since the pressure at this working connection a , b , in operation , is normally higher than at the tank connection t , this connection is also designated as a high pressure connection , while the tank connection t is designated as a low pressure connection . in the same way , the pump valves 11 , 12 have a high pressure connection p which communicates with the pump 6 , and a low pressure connection which communicates with the working connections a , b of the motor 2 . the designations &# 34 ; high pressure connection &# 34 ; and &# 34 ; low pressure connection &# 34 ; hereby serve only to differentiate between the two connections . situations may very likely arise in which the pressure at the low pressure connection is greater than that at the high pressure connection . in the pump valves 11 , 12 , the insert 26 has two channels 29 , 30 in each of which a non - return valve 31 , 32 is arranged . both non - return valves open away from the connections , and therefore block a back flow of fluid into the high pressure or low pressure connection . the non - return valves 31 , 32 are each formed by means of a spring - loaded stainless steel ball , which presses itself into a valve seat in the insert 26 . since the insert 26 , as mentioned above , is formed of plastics material , the ball can here , after a short time , shape the valve seat in such a way that the non - return valves are to a great extent leaktight . both closure elements 20 , 21 have a throttle channel 33 , 34 , the throttle channel 33 being arranged between the non - return valves 31 , 32 and a pressure chamber 35 in the case of the pump valves 11 , 12 , while the throttle channel 34 is arranged between the channel 28 and a pressure chamber 36 in the case of the tank valves . in a manner which is not shown , in the control valves 15 , 16 there are also throttles leading to the tank , but in any case with a greater resistance than in the closure elements 20 , 21 , so that the corresponding control pressure can build up . in series with the tank valves 13 , 14 is arranged a further non - return valve which opens away from the pressure chamber 36 and which is formed of a stainless steel ball 37 , which is pressed , loaded by springs , into an insert 38 of plastics material which is fixedly arranged in the valve housing 17 . the springs of the non - return valves should be weaker than the springs 22 , 23 , of the closure elements . as revealed by fig2 the tank valves 13 , 14 and the pump valves 11 , 12 are offset axially with respect to one another , so that arranging the conduits without oblique bores is made easier . the closure elements 20 , 21 are guided without seals , therefore seal - lessly , in the bushings 18 , 19 . in order to make this state of affairs clear , in fig1 a gap s is therefore shown between the guides , formed by the bushing 18 , 19 , and the closure elements 20 , 21 . in the diagrammatic view of fig1 this gap s is shown greatly exaggeratedly . in reality it is much smaller . in any case , however , it is of a size such that water , which is here used as hydraulic fluid , can flow past the respective closure element 20 , 21 . since the pressure chambers 35 , 36 , which in order to differentiate between the two pump valves 11 , 12 and the two tank valves 13 , 14 are in each case provided with the suffixes a and b , are in each case connected to one another in pairs , it could happen that , in the case of the seal - less construction of the valves 11 to 14 , fluid with pump pressure flows past the closure elements 20 of the pump valves 11 , 12 into the pressure chamber 35 and thence into the pressure chamber 36 of the tank valves 13 , 14 . since the closure elements there are also mounted seal - lessly in the bushes , the pressure could then pass into the high pressure connection , that is to say , supply pressure to the working connections a , b of the motor 2 . the non - return valves 37a , 37b , however , prevent the pressure from the pressure chamber 35 from advancing into the pressure chambers 36 . by this means , despite the seal - less guiding of the closure elements , a leaktight 4 / 3 - way valve is obtained . the 4 / 3 - way valve 1 operates in the following manner : water , which is conveyed by the pump 6 into the pump connection p , flows past the closure elements 20 , 21 into the pressure chambers 35 , 36 , which are connected to one another in pairs in each case by way of a pressure control line 40a , 40b , and thus presses the closure elements 20 , 21 onto the valve seats 24 , 25 . all two - way valves 11 , 12 , 13 , 14 are thus closed . the fluid can here pass on two paths into the pressure chambers 35 , 36 , on the one hand through the gap s , the amount passing through being only relatively small here . the major part of the fluid will pass through the throttle channel 33 , 34 . if it is now desired to move the motor 2 in a certain direction , for example to raise the load 3 in the diagrammatic view of fig1 the pressure in the working connection b of the motor 2 must be increased . for this purpose , the control valve 16 is opened . the pressure chambers 35b , 36a are now relieved of pressure , since the outlet of the control valves 15 , 16 communicates with the tank t . by means of the pump pressure prevailing in the high pressure connection of the two - way valve 12 , the closure element 20b is raised from the valve seat and water with the pump pressure flows into the working connection b . it is of course a pre - requisite here that the closure element 20 has a pressure engagement surface on which the pressure in the high pressure connection can act . during the upward movement of the load 3 , water is now displaced into the working connection a , passes to the high pressure connection of the two - way valve 13 , and there raises the closure element 21a , so that the displaced water can flow back to the tank 7 . the pressure decrease in the pressure chamber 36a is possible because the non - return valve 37a opens in the direction of the control valve 16 . if the operation of the motor is now to be terminated , the control valve 16 closes again , so that the pressure in the pressure chambers 35b and 36a increases . in the case of motors which , as in the case illustrated , may have differing pressures at their working connections a , b , for example , differential cylinders , it may now happen that the two - way valve 13 closes , while the two - way valve 12 is still open . the pump valves 11 , 12 are always supplied with the full pump pressure , while the tank valves open only wide enough to allow through the fluid flowing back from the working motor . since the pressure at the working connection a is higher than at the working connection b , it could now happen that the pressure in the working connection a opens the two - way valve 11 . since , however , the two non - return valves 31 , 32 are arranged in the pump valves 11 , 12 , the higher of the two pressures in the connections separated by means of the valve seat 24 always advances into the pressure chamber 35 , in order there to produce the necessary closing force . also in the case of the differential cylinder shown as the motor 2 , therefore , reliable control is possible , even when water , which has no lubricating properties , is used as hydraulic fluid . the non - return valves 32 furthermore have a certain importance from the safety point of view , in a case in which the following conditions prevail : the valve 1 is in the neutral position , the motor 2 is under load , the non - return valve 10 is not very fluidtight , and the pump 6 is switched off or is defective . in this case , the fluid would flow away from the working connection which was under pressure , directly to the pump connection . the load would descend very rapidly . if the non - return valve 32 is now built into this connection , there is still a connection , by way of the gap s , between the working connection and the pump connection . the fluid flow , however , is kept considerably smaller here .