Abstract: The present invention relates to a double-acting hydraulic pump comprising a piston moving with a reciprocal rectilinear movement inside a cylinder of which each chamber is provided with a suction valve and a delivery valve, the rod of said piston being adapted to be driven by a variable-displacement driving member. The said piston rod extends beyond the piston and moves, outside the cylinder within a space of variable volume, provided in the body of the pump, which communicates, with a member controlling the by-pass device through which the two chambers of the cylinder communicate.