Abstract: A valve assembly is provided, including at least a first valve unit and a second valve unit. The first valve unit is movably mounted within the second valve unit, and includes a first valve housing defining a first valve volume. The second valve unit includes a second valve housing defining a second valve volume different from the first valve volume. The second valve housing includes at least one first port and at least one second port. The first valve unit includes a first valve element reciprocably movable with respect to the first valve seat to selectively open and close a first fluid path through the first valve volume, and a first mechanical biasing element. The second valve unit includes a second valve element reciprocably movable with respect to the second valve seat to selectively open and close a second fluid path through the second valve chamber, the second valve element being affixed to first valve housing for reciprocal movement therewith; and a second mechanical biasing element.