Check level of brake liquid in a feeding tank of the main brake cylinder which should be between labels of "Min" and «Мax». Gradual fall of level of brake liquid in a tank occurs because of wear of overlays of brake shoes. If level of brake liquid quickly goes down for the short period of time, inspect tightness of hydraulic brake system. After any repair or air removal from brake system add level of brake liquid to the label «Мax».