The leading/trailing drum brake is a type of drum brake design. It is typically found on the rear of automobiles, though it has appeared on the front of some older models such as the Bedford CA.[1] Its advantage is that is equally effective whether the vehicle is travelling forwards or in reverse.[2] It is not as powerful as a twin leading design, but that is not so important on the rear where too much braking force could cause the rear roadwheels to lock.
The term "leading/trailing" is used because there is one shoe that is "leading", moving with the direction of rotation of the drum and thus exhibiting a self-applying, or self-servo effect, i.e. being dragged into the friction surface of the drum and thus achieving greater braking force.[2] The other shoe is "trailing", moving against the direction of rotation, being thrown off the friction surface of the drum and not retarding the drum quite so effectively.
When the vehicle is moving in reverse, the role of the leading and trailing shoes is reversed. What would be the leading shoe when the vehicle is travelling forwards becomes the trailing shoe, and vice-versa.[2]