A device comprised a master cylinder driven by a control member (20), a pneumatic booster (14), a piston (22), first (VE1) and second (VE2) balancing valve elements, first (VA1) and second (VA2) intake valve elements, and a tubular support (44) provided with two parts (44A, 44B) move relative to one another to adjust the length of the support (44). The balancing valve elements (VE1, VE2) are borne by a balancing part (44B) of the support and the pneumatic piston (22). The intake valve elements (VA1, VA2) are borne by the control member (20) and an intake part (44A) of the support. The two parts (44B, 44A) of the support are connected to one another by a spring (54) and arm (56) arrangement for adjusting the separation thereof as a function of the power of the force applied to the control member (20).
