Abstract: The invention relates to improved jack-pump devices comprising a pump with a sliding piston forming first and second pump chambers, one on each side, a jack with a sliding piston forming first and second jack-chambers, one on each side, a first valve between the pump chambers, acting unidirectionally from the second to the first pump chambers, a second valve between the first chambers of the pump and the jack, and a communication device between the second chambers of the pump and the jack, the pump and jack pistons being slidably mounted and axially aligned in the same body filled with fluid, a fixed transverse partition disposed in the cylinder body between the two pistons, and a plunger for compensating variations of volume, slidably mounted in one of the two second chambers, close to the corresponding extremity of the body.The main application is as a lifting device for regulating the height of members or parts of moving machines, of seats for chairs, vehicles, building stays, etc.