A control valve assembly is provided for a fluid treatment system, including a housing having a top portion and a bottom portion secured to the top portion, the housing including an inlet and an outlet. At least two modular chambers are secured in the housing. A first chamber is configured to receive fluid from the inlet and a second chamber is configured to provide fluid to the outlet. A piston is also provided which includes a shaft with a plurality of sealing rings. The piston extends through the housing and through the first chamber and the second chamber. The piston is configured to reciprocate in an axial direction to control the flow of fluid in the control valve assembly.