This page shows the detail of the hydraulic regulator in the letter-printing telegraph described here.
Note that the entire apparatus is water-filled. The center piston and connected parts all move up and down, but the surrounding parts are all fixed in place. The openings marked b are a valve which only allows water to flow in and blocks outward flow. When the center shaft is pulled up, these b openings allow water in and there is no resistance to the upward motion. When the shaft is pushed down, these b valves close. Water can escape through the incomplete seal formed at the points marked d, but provides a lot of resistance.