Working principle of drum brake: Drum brake is mainly composed of brake base plate, brake drum, brake shoe, return spring, compression wheel and other parts. When braking, by stepping on the foot pedal and using the lever principle, the force of the push rod is applied to the brake master cylinder. After the pressure of the brake fluid is amplified and pushed the brake slave cylinder, the pistons at both ends of the brake slave cylinder will simultaneously push the same force to the shoe ends of the left and right brake shoes, and the other ends of the two brake shoes are supported by the support rod. At this time, the two brake shoes expand outward and fit with the inner surface of the brake drum to form friction, so as to achieve the purpose of braking. As the wheel is a rotating brake drum, the pressure acting on the brake shoe is asymmetric from left to right, resulting in self increasing and self reducing force. The friction torque of the force increasing brake shoe is 2 ~ 2.5 times that of the force reducing brake shoe, which causes the wear degree of the two brake shoes to be different.