Abstract: An injection molding machine having an extruder, a nozzle, and an housing mounted between the extruder and nozzle. The housing encloses a pressure assist nozzle and a fluidic passage connecting the extruder and the extruder nozzle. A bore within the housing extends into the fluidic passage to define a portion of the passage between the extruder and the extruder nozzle. The bore has an aperture within a side wall which acts as a valve opening within the passage which, when closed, prevents flow between the extruder and the nozzle. An annular sleeve is reciprocated within the bore between a retract position, in which the annular sleeve is withdrawn from the passage, and an extend position, in which the annular sleeve is projected into the passage so as to close the aperture. A plunger piston reciprocally resides within the annular sleeve. The plunger piston is held in a retracted position by the annular sleeve while the annular sleeve is in its retracted position.