The invention relates to an adapter and to a filling method giving the possibility of flexibility in the transfer of fluids from a filling cell to a fluidic circuit, depending on the different filling levels required on a production line, notably finding application for the transfer of fluids such as coolant fluid, brake fluid, air-conditioning fluid, to the corresponding fluidic circuit of a vehicle, for example on an automotive assembly line, or further in the energy sector for filling electric radiators with a fluid.The adapter comprises a conduit 4 connected to the filling cell 16 for sucking up filling fluid 5 contained in, and filling with the filling fluid 5, the fluidic circuit 2, and a plunger tube 6, secured to a piston 7 axially adjustable on an axis Y in a first guiding tube 9, having first and second ends 14, and connected through the first end to the conduit.The piston forms with the first guiding tube a first chamber able to receive a movement transmission liquid 3 through a hydraulic transfer line 10 in order to allow, by injection or by suction of the movement transmission liquid into, or from the first chamber, the movement of the plunger tube, via the piston, into a first or a second direction, along the axis Y, for plunging into or moving upwards the fluidic circuit.