Abstract: There is provided a wire line core drilling apparatus including an elongated assembly which is hydraulically propelled along the interior of a drill string to a desired location adjacent the bit end of the drill string under the influence of a moving pressurized liquid within the drill string, characterized in that said elongated assembly is provided with an annular cup seal having a flexible annular lip thereon which expands radially outwardly under the influence of liquid pressure thereon to substantially prevent by-pass of the pressurized liquid between the elongated assembly and the interior wall of the drill string and which annular lip is capable of expanding and contracting as said assembly moves along the drill string interior to accommodate reasonable fluctuations in drill string interior diameter which may be encountered.