Abstract: An expanding chamber hydraulic motor venting through a working member reduces the motor/stroke-limiting venting valve assembly to three simple components: a cup-shaped cylinder, an annular gasket at the open end of the cylinder, and undersize piston sufficiently loose in the cylinder to provide free-flow of hydraulic fluid therebetween but protruding through the gasket in a sealing fit; in operation, fluid pressure introduced by conventional means between the piston and cylinder can force the piston to extend from the cylinder only within a maximum stroke length limited by an annular groove around the piston which when it reaches the gasket vents the pressure past the gasket.