A servobrake (10) including a pneumatic brake booster (12) and a master cylinder (22) with a main piston (20) that slides in a front pressure chamber (24). The main piston (20) has a fixed first rear end (46) acted on by a pneumatic piston (16) of the booster (12) and a moving second rear end (48) acted on by a feeler (18) of the booster (12) that is a coaxial with the piston (16) and acts upon a reaction ram (52) to communicate pressure forces in a front pressure chamber (24) of the master cylinder to the second end (48). A hydraulic isolation valve shutter (58), independent of the internal piston (50) of the ram (52), is activated when the input force of the booster (12) exceeds a given rate threshold so that the braking force is not reduced.