A servo valve comprises a valve body, an elongated chamber defined by the body and two oppositely facing annular pistons independently movable longitudinally within the body. Each piston has an inner annular first valve seat and an outer annular first obturating surface. A reciprocable valve stem is surrounded by the pistons. Two oppositely facing spaced annular second obturating surfaces are carried by the valve stem a fixed distance apart, with the pistons located between the two second obturating surfaces and reciprocable on the valve stem. Each of the second obturating surfaces is arranged to cooperate with one of the inner annular first valve seats to control fluid flow along the valve stem from a fluid inlet intermediate the pistons. Two annular second valve seats are fixed in the valve body. Each of the annular second valve seats is located to contact the outer annular first obturating surface of one of the two pistons to limit the longitudinal movement of the pistons away from each other and to control fluid flow past the outer annular first obturating surfaces. The pistons each include a surface for transmitting force to the piston in a direction away from the other piston.