The truck of a railroad car or similar vehicle is provided with a hydraulic fluid single action piston and cylinder motion damping device located in each spring group, between the spring seat and the truck bolster. The cylinder is divided by the piston head into a variable volume compression chamber under the piston head and a variable volume compensating chamber over the piston head. The device is also provided with unvalved passages by means of which hydraulic fluid is by-passed around the piston head between the variable volume compression chamber under the piston head and the variable volume compensating chamber over the piston head, and with valved passages by means of which hydraulic fluid flows back and forth between the compression chamber and a reservoir. Within the compression chamber is a purge valve by means of which air is forced out of the cylinder back into the reservoir through the valved passages in response to retraction of the piston.