The valve assembly includes a main valve body defining a first inner cylindrical cavity, a support plate positioned above the main valve body, and an upper valve head positioned above the support plate. The upper valve head includes a bottom planar bottom surface and a plurality of discrete conduits, each conduit extending between an outer end and an inner end. The valve assembly further includes a sealing diaphragm interposed between the support plate and the upper valve head. The diaphragm can be engaged by plungers to selectively open or close corresponding gas circuits.