A hydraulic mine prop comprises a cylindrical top ram portion and a bottom ram portion which is telescopically engageable in said top ram portion and has an upper piston portion in sliding sealing engagement with the interior of said top ram portion. A ram head closes the top of said ram portion and a valve seat body is disposed between the ram head at a spaced location therefrom adjacent the top of the top ram portion and defines a vent space therebetween. The valve seat body also encloses a pressure space between itself and the piston and a passage is defined through the valve body within a valve seat portion thereof which communicates the pressure space to the vent space. The passage is closed by a spring-biased valve which may open against the biasing force of the spring. The construction includes one or more passages extending through the valve seat and through various generally radially extending passages defined in the body from the pressure space to the vent space and additional passages which are defined outwardly from the vent space to the atmosphere. The construction also includes a ring around the top ram portion having an annular passage which is communicatable through a port or bore of the top ram portion cylinder to a pressure holding valve space which is closable by a rotatable globe valve.