The invention discloses a fluid system for an article of footwear or other products, concretely a fluid system with a expansible pump cavity . In one aspect of the invention, the fluid system includes a pump chamber and a pressure chamber. The pump chamber is formed to exhibit a four layer structure to imparts an expandable configuration. The four layers are bonded to each other such that the sidewall has a zigzag-shaped configuration. In another aspect of the invention, the fluid system includes two pump chambers and a pressure chamber in order to increase the resulting pressure in the pressure chamber. In yet another aspect of the invention, at least one of the two pump chambers has an expandable configuration.