Abstract: A pump has a rotatable rotor carrying a plurality of radially-extending pistons bearing at their outer ends on an annular adjustment ring which can be displaced radially in the pump housing by means of a pair of diametrically opposed adjustment cylinders that serve to displace this ring and adjust the stroke of the radial pistons in the rotor. One of the adjustment pistons is of larger diameter than the other and both of the adjustment pistons are spring-biased against the outer periphery of the adjustment ring and both have throughgoing bores so that pressure behind these adjustment pistons is fed to pressure-relief pockets at their inner ends against the adjustment ring. The small-diameter piston is provided in its through-going passage with a pressure-limiting valve which only allows pressure past to its respective pressure-relief pocket when pressure behind the smaller piston exceeds a predetermined minimum which is approximately a tenth of the maximum pump operating pressure.