How long does this system need to main pressure? I have concerns about low level leaks around o ring seals inside typical readily available pipes and tubes. Especially if you want a reasonable amount of friction so the weight actually moves in the tube. I can imagine the friction getting out of hand trying to maintain a seal, the rod having to be increased in length to compensate, then the friction becoming a bit inconsistent such that you get a situation where the rod doesn't move... until it does, then pressure spikes up rapidly.
But I'm just concerned. It might turn out OK.