Makes sense but it's not entirely following the technical manual explanation "fluid enters from the top creating a fluid seal". If the bellows were used for the spring aspect I would question the material choice, brass has a very low module of elasticity, you don't see brass springs. I managed to restore 2 pumps out of 3 so far and when I extend the bellows there is very little spring back, I am more in favor for the bellows being used for the reverse "chinese finger trap" capacity.
Not a big deal, after all, it works.