Mixing chamber assembly having a reciprocating rod within a mixing chamber of a mixing chamber member. The rod is in a non-interference position with the mixing chamber over its entire length within the mixing chamber. The non-interference fit provides continuous solvent flow about the exterior of the rod and interior of the wall defining the mixing chamber so solvent drips out the discharge end of the mixing chamber and the rod is free to float or ride on a solvent film. "Solvent" can include a combination of solvent and other ingredients such as lubricants, emulsifiers and flow control particles. The rod's ability to ride on a solvent film over the entire portion received within the mixing chamber, allows for a much reduced reciprocating motive force requirement and a corresponding freedom to increase the mixing chamber volume. The mixing chamber member is preferably received within a dispenser housing such that it is free to shift between dispensing and non-dispensing modes with a shuttle valve arranged between the housing and mixing chamber to seal off foam chemical precursors from entering the mixing chamber when in a non-dispensing mode. Solvent flushing preferably includes solvent flow internally with the mixing chamber through the precursor inlet ports when not feeding the chemicals, and through a channel between the mixing chamber member and dispenser housing. A dispensing system is also provided wherein an intermittent, torque-based, and AC motor driven pump provide chemical precursors on demand, rather than continuously.