Abstract: For use in servicing a completed well exposed to the possibility of upward flow, an improved tool is set forth in the preferred and illustrated embodiment. This tool is adapted to be run on a wireline to support wireline service equipment therebelow; it incorporates a set of weights which are affixed to a sleeve, the sleeve connecting with a set of serrated collet slips on a tapered cone. The tapered cone is connected with a central rod or stem which extends therebelow, telescoping in the sleeve, the two being forced to telescope by a coil spring. At the top end, the cone is connected with a fishing neck and rope socket to enable the device to be run on a wireline. If flow from the bottom of the well occurs, the tool is forced to an expanded position and thereby sets, substantially blocking the passage to limit fluid flow pass the tool.