A mixing chamber assembly which has a reciprocating rod (
) received within a mixing chamber of a mixing chamber member (
). The reciprocating rod (
) is in a non-interference position with the mixing chamber over its entire length within the mixing chamber. The non-interference fit provides for continuous solvent flow about the exterior of the rod and interior of the wall defining the mixing chamber such that solvent drips out the discharge end of the mixing chamber and the rod is free to float or ride on a film of solvent. The ability for the rod to ride on a film of solvent 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 behind a dispensing and a non-dispensing mode with a shuttle valve arranged between the housing and mixing chamber to seal off chemical precursors for forming polzurethane foam or the like from entering the mixing chamber when the mixing chamber member is in a non-dispensing mode.