The invention relates to a primary spring device (100) for a tandem master-cylinder unit, which is to be installed between the primary piston and the secondary piston of the tandem master-cylinder unit. The device (100) includes a plastic frusto-conical abutment (110), the thickness of which is not constant. The thickness of the wall of the body of the abutment at the apex (112) and adjacent to the apex is increased relative to the remainder of the wall. Longitudinal ribs (125) for stiffening and for guiding the head (141) of the screw (140) are provided on the inner surface near the apex and over a certain length of the body. A compression coil spring (150) is supported between the outer abutment collar (115) of the base (114) of the abutment (110) and a bearing disc (144) installed to as to be adjustable by means of a nut (143) at the end of the body (142) of the screw.