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 term "solvent" can include a combination of solvent and other ingredients such as lubricants, emulsifiers and flow control particles. 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 polyurethane foam or the like from entering the mixing chamber when the mixing chamber member is in a non-dispensing mode. A spring can be utilized to maintain the mixing chamber member in a non-dispensing state while a piston arrangement record with a piston chamber formed in the back of the dispenser housing can be used to reciprocate (at preferably different times) both the mixing chamber member and purge rod to the desired positions. A dispensing system is also provided wherein an intermittent, torque-based, and AC motor driven pump to provide the chemical precursors on demand, rather than continuously. Heated conduits are also employed to conduct the pumped chemicals to the dispenser.