to rebuild or replace ?

Well, although I thought I had another one around the size of these 2, another 4" one.... But the 2 that I can readily put my hands on are a 4x18 and a 3x20 and a couple of more that are smaller.
These are "cylinder only" dimension, not included are the ram that sticks out beyond that, or the round piece of pipe that is on the butt end for a mount..... I can say pretty definitely the red one is Brand new, (4x18) and the 3x20 I'm pretty sure is too, the fitting someone screwed into it definitely looks older than the cylinder. I don't think that one has been used (I know I haven't) but can't guarantee that.