The drum brake uses brake shoes that have friction material called linings attached to them. Linings can be riveted or more often bonded to the brake shoes. Most linings have a manufacturer's code on the edge of the lining which will identify their specified co-efficient of friction.
The composition of the friction material affects brake operation. Linings which provide good braking with low pedal pressures tend to lose efficiency when they get hot. This means the stopping distance will be increased. Linings which maintain a stable friction co-efficient over a wide temperature range, generally require higher pedal pressures to provide efficient braking, and may need the use of a booster.
This friction material was once made of asbestos but concerns about health problems associated with asbestos have led to the use of non-asbestos alternatives. However, if you are removing older brake linings it is impossible to tell whether asbestos was used as a component in their manufacture, so they must be handled with care. Wear protective clothing and make sure you do not inhale any of the brake dust residue, and very importantly, never use an air hose to blow brake off any component.