Rotation damper (1), for a vehicle that runs on rails, having piston (9) and cylinder parts (2) moving with respect to one another. This damper has a friction characteristics, that is to say the fluid connection over the piston is not a permanently open hole but exclusively comprises spring-loaded valves (11). In order to facilitate the installation of such dampers, it is proposed to make a separate connection between the working cylinder and the reservoir in which a valve that can be manually operated is arranged, which valve, on operation, opens a connection as a result of which the piston and cylinder can be moved relative to one another under manual force.