Prior to testing and probably painting, the last stage of manufacturing a gas spring or gas strut is pressurizing with Nitrogen. I was wondering how this step is done. I think it is filled with gas from the rod side and the rod seal works as an one way path, meaning, nitrogen gas is injected into the cylinder with certain amount of pressure, and then when the injection is stopped, the pressurized nitrogen in the cylinder pushes the seal back and seals the gas strut, but I’m not sure.
See https://youtu.be/w0XYrs8_NFc?t=302 at 5:02. The whole gas spring is placed inside a pressure vessel, which is filled with gas at high pressure. As you suggest, the seal around the rod acts as a one-way valve, letting gas in, but not back out again.