The brake shoe is a curved piece of metal with high-friction material (the brake lining) attached to the outside of it. It is attached to the backing plate inside the brake drum with a series of springs and levers, and rests against the hydraulically operated wheel cylinder.
When the driver presses the brake pedal, hydraulic fluid is forced through the brake pipes and into the wheel cylinder, which expands, pressing the brake shoe outwards against the inner wall of the rotating brake drum. The resultant friction is what slows down the vehicle.
The spring-and-lever system performs two functions – it automatically adjusts the brake shoe to compensate for wear, and it is also linked to the handbrake.
The brake lining is made of a high-drag material that includes graphite, powdered metals such as lead, zinc and aluminium, and also crushed up cashew nut shells.
Unfortunately, drum brakes can be susceptible to heat generation, which can have an adverse effect on braking performance.
If the drum is subjected to a long period of hard braking the heat generated can cause the drum itself to expand, meaning that the brake shoes have to be pressed farther outwards to press against the drum’s inner surface.
This gives the car what is termed as a ‘long pedal’, because the driver has to press it farther down to get the same braking performance.
Drum brakes can also be susceptible to water ingress. If water gets into the drum it can end up being compressed between the brake shoe and the inside of the brake drum, and will have an adverse effect on the length of time it will take a vehicle to stop.
Unlike a drilled disc brake, there’s nowhere for the water to go, so it will continue to affect braking performance until sufficient heat is generated to vapourise the water. Only then will full braking be resumed.