Check the level of brake fluid in the feeding tank of the main brake cylinder which has to be between tags of "Min" and "Max". Gradual lowering of the level of brake fluid in a tank happens because of wear of overlays of brake shoes. If the level of brake fluid quickly goes down for the short period of time, carry out an inspection of tightness of the hydraulic brake system. After any repair or removal of air from the brake system add the level of brake fluid to the tag "Max".