Abstract: A pipet gun for drawing liquids into and dispensing liquids from a pipet which includes a variable flow valve assembly. The valve assembly includes a valve which is capable of accommodating a plurality of flow rates, and which is progressively adjustable during pipetting between a first air flow rate and a second air flow rate which is in excess of the first flow rate and which is non-progressively adjustable during pipetting to a third air flow rate which is substantially greater than the second air flow rate. The valve assembly further includes a threshold detector in the form of a spring detent, which provides an indication to the operator that the pipet gun is operating at the increased fluid rate, i.e., the “express mode” of operation.