A hydraulic servo-control of a servo-controlled gearbox provided with a hydraulic accumulator, having an outer housing and a piston that is slidable axially inside the outer housing and adapted to define a first variable-volume chamber for a gaseous material and a second variable-volume chamber for a control fluid under pressure; and at least one limit stop arranged at an open end of the outer housing which serve as a lower limit stop element; wherein the outer housing is provided with an upper limit stop element, which acts to limit the pressure value inside the first variable-volume chamber within a predetermined safety range.

An expansion tank for at least temporarily storing a pumped liquid under pressure, the including a thin wailed outer shell and a diaphragm located internally of the tank, and sealingly secured to the inner surface of the shell of the tank to divide the internal volume of the tank into a fluid-tight section for holding a gas under pressure and a fluid-tight section for holding a liquid under pressure. The diaphragm has an enlarged lip around the outer circumference of the diaphragm and being is connected to the interior portion of one of the substantially cylindrical sections via the enlarged lip being fitted tightly into a coupling ring, which in turn presses the enlarged lip of the diaphragm sealingly, circumferentially against the inner circumferential surface of the tank wall when the outer circumferential surface of the coupling ring is sealingly connect to the inner wall surface of one of the tank segments, sealing the enlarged lip against the inner surface of the tank. The tank segments are not finally assembled until after the diaphragm is sealed against the tank wall, so that the diaphragm divides the interior volume of the tank into two mutually fluid tight volumes that can be rendered fluid tight with respect to the space outside of the tank walls.