Diaphragm joint and convolutions, bottom screen diffuser, air stem, cap, and diaphragm restrictor systems for a pressure vessel are disclosed. A convoluted diaphragm divides the vessel into a pair of sealed chambers. The convoluted geometry of the diaphragm minimizes stress on the diaphragm at maximum displacement conditions. An H-ring, with or without being over-molded by the convoluted diaphragm, may be configured to receive end portions of the tank liners. A bottom diffuser, coupled to an inlet of the vessel, diffuses and mixes water flowing into and out of the vessel and drains water out of the vessel. Fiberglass windings surround and lock the tank liners in tension. The cap system includes a valve cap that engages an air stem. An outer cap covers a recess of the vessel and includes a hollow cavity that receives the valve cap. A diaphragm restrictor limits upward movement of the diaphragm within the vessel.