Abstract: There is disclosed a valve having a spool sealably slidable longitudinally within a bore in its body between alternate positions for controlling flow of a first fluid between lateral ports connecting with the bore. The spool is urged toward a first position by a force due to a spring arranged within a chamber at one end of the bore, and to a second position by a force due to the pressure of a second fluid acting across a piston reciprocable within a chamber at the other end of the bore. A conical shoulder on a head of the spool engages a conical seat on the valve body at the intersection of the other end of the bore with the piston chamber so as to locate the spool in its second position and form a metal-to-metal seal with the seat.