the passage 15 the piston 23 is guided on both sides by guidance grooves 21 made in the wall of the housing 14 the movement of the piston 23 will take place very quickly in which it should be considered that the pressure used could be approximately 5 bar finally the end 23 a end up in the cavity 20 in which it is accommodated in a snugly fitting manner at that moment the pin 27 has come out of the duct 31 the surface of the smallest passage of the duct 31 little duct 32 is smaller than the one of the nipple 30 so that no discharge will take place before the piston 23 has moved out completely the discharge opening 29 has then just become exposed in the passage 15 which further contributes to the prevention of premature discharge the pressure prevailing in the chamber 19 can no longer have the effect that the piston 23 moves further so that said pressure is entirely used for discharging cleansing medium via chamber 19 access 26 duct 31 little duct 32 and finally discharge opening 29 in the direction c towards the teat cup 5 the piston 23 a kind of sliding cock as it were here completely closes off the passage 15 so that there is no danger that cleansing medium ends up in the milk line 6 the fitting of the piston between the grooves 21 and the cavity 23 is so snug that the u2014 intended u2014 operation movement of the piston is possible though that leakage of liquid past the piston is prevented then the teat cups are removed from the teats but the pressure in.