Coupling part, in particular coupling sleeve, for coupling with a corresponding coupling part, in particular a coupling plug, of a coupling for pressure medium lines, in particular hydraulic lines, comprising - a housing with a continuous first passage extending in the axial direction, - a valve tappet, which is coaxially arranged in the first passage of the housing, - a sleeve inner body, which is coaxially arranged in the first passage of the housing resting thereagainst in certain regions and which has a cylindrical guide section, by which a central second passage is delimited, which is arranged coaxially to the first passage, - a valve sleeve, which is displaceably arranged in the axial direction between a first position and a second position relative to the sleeve inner body, through the guide section of which it is axially guided along the inner surface of the guide section on an end-side sliding section of the valve sleeve, wherein the valve sleeve seals against the sleeve inner body in the first position as well as in the second position, and seals against the valve tappet in the first position and delimits a gap between the valve tappet and the valve sleeve in the second position, wherein, in the first position of the valve sleeve, a flow channel is delimited by the sleeve inner body, the valve sleeve and the valve tappet, wherein the valve sleeve has a circumferential shoulder on the inside, which opens into a hollow cylindrical end section, wherein the inner diameter of the hollow cylindrical end section is slightly smaller than the outer diameter of the sliding section of the valve sleeve, characterised in that the amount of the difference between the inner diameter of the hollow-cylindrical end section of the valve sleeve and the outer diameter of the sliding section of the valve sleeve is less than eight tenths of a millimetre and/or in that the circular area enclosed by the inner diameter of the valve sleeve is smaller by less than 12 mm 2 than the circular area enclosed by the outer diameter of the valve sleeve. - a housing with a continuous first passage extending in the axial direction, - a valve tappet, which is coaxially arranged in the first passage of the housing, - a sleeve inner body, which is coaxially arranged in the first passage of the housing resting thereagainst in certain regions and which has a cylindrical guide section, by which a central second passage is delimited, which is arranged coaxially to the first passage, - a valve sleeve, which is displaceably arranged in the axial direction between a first position and a second position relative to the sleeve inner body, through the guide section of which it is axially guided along the inner surface of the guide section on an end-side sliding section of the valve sleeve, wherein the valve sleeve seals against the sleeve inner body in the first position as well as in the second position, and seals against the valve tappet in the first position and delimits a gap between the valve tappet and the valve sleeve in the second position, wherein, in the first position of the valve sleeve, a flow channel is delimited by the sleeve inner body, the valve sleeve and the valve tappet, wherein the valve sleeve has a circumferential shoulder on the inside, which opens into a hollow cylindrical end section, wherein the inner diameter of the hollow cylindrical end section is slightly smaller than the outer diameter of the sliding section of the valve sleeve, characterised in that the amount of the difference between the inner diameter of the hollow-cylindrical end section of the valve sleeve and the outer diameter of the sliding section of the valve sleeve is less than eight tenths of a millimetre and/or in that the circular area enclosed by the inner diameter of the valve sleeve is smaller by less than 12 mm 2 than the circular area enclosed by the outer diameter of the valve sleeve.