The drum brake is equipped with an automatic adjuster which adjusts the brake shoes towards the brake drum when lining wear occurs.
The adjustment mechanism is equipped with a bi-metallic strip which prevents undesired adjustment of the brake shoes when the brakes are run hot.
The heat generated during braking is passed to the bi-metallic strip. If the heat exceeds a critical point, the bi-metallic strip bends upwards into the corresponding nib and prevents further adjustment.
After the brake has cooled again, the bi-metallic strip returns to the initial position. Adjustment is now possible again.