An apparatus for use in turning steerable vehicle wheels includes a first pump driven by an engine of the vehicle to supply fluid under pressure to a power steering motor assembly through a supply conduit. A second pump driven by the engine of the vehicle supplies fluid under pressure to the power steering motor assembly when the flow of fluid in the supply conduit is below a predetermined value. The second pump is inactive when the flow of fluid in the supply conduit is above the predetermined value. The second pump has vanes located in slits. A valve which is in fluid communication with the slits has a first condition in which fluid under pressure is supplied to the slits. The valve has a second condition in which the slits are in fluid communication with a reservoir.