The hydraulic pressure generated by a master cylinder and servo amplifier (PG) is transmitted to the wheel brake cylinder (WC) through the adjuster (FV) connected to a container (RT) storing a predetermined quantity of fluid from previous application and release of the brake. The cylinder pressure is increased stepwise by pumping (FP) through a return line (RP). The content of the container is estimated (ER) from the amt. supplied (computed from the pressure in the cylinder at the commencement of its redn.) and the amt. extracted (from the pressure at the commencement of its stepwise increase and in response to the stepping time).
