Abstract: Three elongated tubular members are disposed within one another, the second member being spaced inwardly of the first member to form a fluid channel therewith and sealed and extending through a transverse wall dividing the first member into two chambers, the third member being in sliding engagement within a passageway in the second member communicating with ports through the second member on opposite sides of the transverse wall in communication with the channel. The third member is provided with a recess communicating with the ports. The first member has two open ends, the second member has an open downstream end and closed upstream end and both opposite ends of the third member are closed.