The valve stem is the component responsible for dispensing the soda water. It consists of three parts: the plastic stem itself, a sealing ring (O-ring) and a rubber seal (pill rubber) at the end. It is the most important part of the siphon head and the one that is subjected to the most frequent wear.
The plastic stem rarely fails, although it is subject to wear due to the movement of the lever. But even a 6-piece repair kit includes a new plastic stem, which is probably not a coincidence.