In a car brake drum, there is generally only one wheel cylinder, when the wheel cylinder is subject to the hydraulic force from the total pump, the piston at both ends of the wheel cylinder will simultaneously push up to the left and right foot of the brake shoe, acting force is equal. But because the wheel is rotating, brake drum acting on the brake shoe pressure left and right asymmetrical, resulting in self-increasing or self-reducing force. Therefore, the industry will increase the force of the side of the brake shoes called collar hooves, reduce the force of the side of the brake hooves called from the hooves, the friction torque is from the hooves of 2 ~ 2.5 times, two brake shoes friction lining wear degree is not the same.

In order to maintain good braking efficiency, there should be an optimal clearance value between brake shoes and brake drums. With the wear of friction lining, the gap between brake shoe and brake drum increases. In the past, drum brake clearance needs to be adjusted manually, with a feeler to adjust the clearance. Now car drum brakes are automatic adjustment mode, friction lining wear will automatically adjust and brake drum clearance. When the gap increases and the brake shoe pushes out beyond a certain range, the adjusting gap mechanism will pull the adjusting rod (pawl) to the joint position with the next tooth of the adjusting tooth, so as to increase the length of the connecting rod, make the brake shoe position displacement, and restore the normal gap.

Car drum brakes are generally used for the rear wheel (front wheel disc brakes). Drum brakes in addition to the relatively low cost, there is a benefit, is convenient and parking (parking) brakes together, the rear wheel for the drum brakes of the car, the parking brake is also combined in the rear wheel brake. This is a mechanical system, which is completely separated from the brake hydraulic system on the vehicle: the use of a manual joystick or parking pedal (American car) to tighten the steel cable, control the drum brake bar extension brake shoes, play a parking brake role, so that the car will not slip; Loosen the steel cable and return the spring to make the brake shoe return to its original position and the braking force disappears.