A reciprocating positive-displacement diaphragm pump for liquids, comprising: at least a piston-cylinder group, having: - a liquid pump chamber, - a piston-cylinder pair, the cylinder of which has a front mouth intersecting the pump chamber and exhibits a cylindrical internal surface (31 b) for guiding the piston, - the piston having a cylindrical lateral surface coupled in axial sliding and with an oil seal with the guide surface (31 b) of the cylinder, - a flexible diaphragm, a perimeter of which is fixed to the internal surface of the pump chamber, which separates the chamber from the cylinder mouth, the diaphragm being connected to the front thrust surface of the piston, - a thrust chamber, external of the pump chamber, delimited between the diaphragm, the thrust surface of the piston and the guide surface (31 b) of the cylinder, - the chamber being sealedly closed and being filled with non-compressible thrust oil, - the piston comprising a front seal edge, which realizes the seal by acting against the guide surface (31 b) of the cylinder, which defines a mobile confine of the thrust chamber, the pump further comprising: - a mechanism for actuating a reciprocating axial movement of the piston in the cylinder, between a bottom dead centre and an upper dead centre, and vice versa, - a rear tank, closed and filled with thrust oil, which closes the rear portion of the piston, separate and not communicating with the thrust chamber, characterised in that the rear tank is delimited, on the front transversal side, by the piston and on the lateral side by a tubular lateral wall, comprising a front portion having an internal surface defining the cylindrical guide surface (31 b) for the piston and a rear portion (41 a) joined to the front portion; and further characterised in that in order to restore the thrust oil level, lost by leaking from the thrust chamber to the rear tank, the tubular wall comprises at least an open groove fashioned on the guide surface (31 b), and facing the lateral surface of the piston, which develops substantially axially, having a rear end connected to the rear tank and a front end which, when the piston is in the environs of the lower dead centre, opens frontally of the front seal edge and communicates with the thrust chamber. at least a piston-cylinder group, having: - a liquid pump chamber, - a piston-cylinder pair, the cylinder of which has a front mouth intersecting the pump chamber and exhibits a cylindrical internal surface (31 b) for guiding the piston, - the piston having a cylindrical lateral surface coupled in axial sliding and with an oil seal with the guide surface (31 b) of the cylinder, - a flexible diaphragm, a perimeter of which is fixed to the internal surface of the pump chamber, which separates the chamber from the cylinder mouth, the diaphragm being connected to the front thrust surface of the piston, - a thrust chamber, external of the pump chamber, delimited between the diaphragm, the thrust surface of the piston and the guide surface (31 b) of the cylinder, - the chamber being sealedly closed and being filled with non-compressible thrust oil, - the piston comprising a front seal edge, which realizes the seal by acting against the guide surface (31 b) of the cylinder, which defines a mobile confine of the thrust chamber, - a liquid pump chamber, - a piston-cylinder pair, the cylinder of which has a front mouth intersecting the pump chamber and exhibits a cylindrical internal surface (31 b) for guiding the piston, - the piston having a cylindrical lateral surface coupled in axial sliding and with an oil seal with the guide surface (31 b) of the cylinder, - a flexible diaphragm, a perimeter of which is fixed to the internal surface of the pump chamber, which separates the chamber from the cylinder mouth, the diaphragm being connected to the front thrust surface of the piston, - a thrust chamber, external of the pump chamber, delimited between the diaphragm, the thrust surface of the piston and the guide surface (31 b) of the cylinder, - the chamber being sealedly closed and being filled with non-compressible thrust oil, - the piston comprising a front seal edge, which realizes the seal by acting against the guide surface (31 b) of the cylinder, which defines a mobile confine of the thrust chamber, - a mechanism for actuating a reciprocating axial movement of the piston in the cylinder, between a bottom dead centre and an upper dead centre, and vice versa, - a rear tank, closed and filled with thrust oil, which closes the rear portion of the piston, separate and not communicating with the thrust chamber,