A sensor feeler device for a brake booster. The sensor feeler device (4) includes an axially moveable cylinder (40), the movement of which can be controlled by the control rod which is itself controlled by the brake pedal, a sensor feeler piston (5) able to move axially in said moving cylinder (40), and a spring (44) that tends to push the sensor feeler piston (5) forward and keep its front face substantially in the same plane as the front face of the moving cylinder.