a sealing sleeve for a gate valve is disclosed which is designed to respond to movements of the gate of the valve in a particular manner . the seal undergoes an axial compressive type movement as the gate is moved between opposed sealing sleeves and this causes a predetermined radially inward buckling of an intermediate wall section of the sealing sleeve . this predetermined buckling creates a spring bias in the sleeve , urging the sleeve to return to the original , less stressed condition . preferably , the sleeve is of a composite structure and a particular component of the intermediate wall section creates this spring bias . this structure has improved operating characteristics and accommodates a wider range of resilient materials to be used for the sealing sleeve .