Abstract: An adjustable distribution valve comprises a body defining first and second axially-aligned branches and a third branch extending laterally thereof, substantially in the form of a T. A plug element is fitted in the passage formed by the axially-aligned branches and is movable therewithin. The plug element is bounded by a cylindrical surface of diameter equal to the diameter of the passage, and two opposed concave surfaces which intersect the cylindrical surface at a point substantially halfway along the plug element and at opposite ends of the plug element at positions which are diametrically opposite said point.