The detector is used to sense the movement of a brake pedal (1) of a vehicle. The pedal is coupled to a first piston (7) which compresses fluid in a first liquid chamber (9) when the pedal is pressed. The first liquid chamber is connected to a second liquid chamber (13) through a small orifice (11). The second liquid chamber has a second piston (15) and a spring (17). The spring acts on the second piston and opposes the pressure built up in the two liquid chambers when the pedal is pressed. The hydraulic pressure in the liquid chambers are sensed by respective pressure sensors (19,20). A control unit (24) controls a master cylinder (22) based on the differential pressure value obtained using output signals of the pressure sensors.
