A coupling part, especially a coupling cuff, for a quick-release coupling for high-pressure hydraulic lines comprising a coupling housing having a through-flow channel with a valve device disposed within the through-flow channel having a tappet-shaped closing valve for closing off the through-flow channel in an uncoupled state of the coupling part; wherein the coupling housing contains a conical valve seat surface and the closing valve has a longitudinal valve axis, a valve stem and a valve head, said valve head having a conical surface and a valve sealing ring for sealing contact with the valve seat surface, wherein the valve sealing ring has a ring circumference surface that projects beyond the conical surface in a radial direction and has a valve seal diameter D VD, wherein the flow-through channel has a ring-shaped cross-sectional widened region having a maximal expansion diameter D EW, wherein the cross-sectional widened region lies radially opposite the ring circumference surface in a maximally open position of the closing valve with regard to the longitudinal valve axis, characterized in that the closing valve, viewed in a direction of the longitudinal valve axis, has a first, head-side valve end and a second valve end, and the valve device has a bearing sleeve at the second valve end, wherein the closing valve is mounted by the bearing sleeve so as to slide and to be displaceable, back and forth, in the coupling housing, in an axial direction, that the coupling part has a locking device for locking the closing valve in an open position, wherein the locking device has: a) a spring clamp, especially a U-shaped one, having two spring arms, b) the bearing sleeve which is configured with - a guide body having radially projecting guide elements, wherein the bearing sleeve is mounted in the coupling housing, and two clamp accommodation recesses that pass through the guide body, wherein one of the spring arms respectively passes through the clamp accommodation recesses, - a guide pipe that follows the guide body at one end and projects away from the guide body in the axial direction, and - a bearing recess that passes through the bearing sleeve in the axial direction, c) a sliding sleeve that is mounted so as to be displaceable, back and forth, in a sliding sleeve guide groove of the valve stem, by a limited amount, in the axial direction, wherein d) the sliding sleeve and the valve stem are each mounted in the bearing recess so as to slide and be displaceable in the axial direction, e) the sliding sleeve, the spring clamp, and the sliding sleeve guide groove interact to lock the closing valve in an open position, and f) the guide body and the guide pipe are configured in one piece or in two pieces. a) a spring clamp, especially a U-shaped one, having two spring arms, b) the bearing sleeve which is configured with - a guide body having radially projecting guide elements, wherein the bearing sleeve is mounted in the coupling housing, and two clamp accommodation recesses that pass through the guide body, wherein one of the spring arms respectively passes through the clamp accommodation recesses, - a guide pipe that follows the guide body at one end and projects away from the guide body in the axial direction, and - a bearing recess that passes through the bearing sleeve in the axial direction, - a guide body having radially projecting guide elements, wherein the bearing sleeve is mounted in the coupling housing, and two clamp accommodation recesses that pass through the guide body, wherein one of the spring arms respectively passes through the clamp accommodation recesses, - a guide pipe that follows the guide body at one end and projects away from the guide body in the axial direction, and - a bearing recess that passes through the bearing sleeve in the axial direction, c) a sliding sleeve that is mounted so as to be displaceable, back and forth, in a sliding sleeve guide groove of the valve stem, by a limited amount, in the axial direction, d) the sliding sleeve and the valve stem are each mounted in the bearing recess so as to slide and be displaceable in the axial direction, e) the sliding sleeve, the spring clamp, and the sliding sleeve guide groove interact to lock the closing valve in an open position, and f) the guide body and the guide pipe are configured in one piece or in two pieces.