A plunger driven solution pump is described that is configured to have a diaphragm that substantially conforms to the volume of a pump solution chamber when in a fully outwardly deflected state. The solution chamber may have an inlet port and an outlet port form in a concave solution chamber wall. A step, or ridge, may be formed along an outer periphery of the solution chamber wall and adjacent the inlet and outlet ports to prevent the diaphragm from becoming deformed from pressure against the solution chamber wall. This configuration may allow the pump to efficiently self-prime.