Auto Questions: How Drum Brakes Work

A glance into the inner workings of a drum brake.

Automobile owners for decades have been baffled by the mystery of the braking system. Many are totally confused when their mechanic tells them they need new shoes, the drums need to be turned and their springs are shot.

Shoes? What's wrong with what I have on? Drums and springs? That sounds like my son's band. What do these things have to do with the squealing noises my car is making?

Seriously, a lot. If your car is making squealing noise every time you stop, you most likely have a brake problem.



First let's look at the braking system. For the most part it's a closed hydraulic system. That is, fluid in the system works a piston that activates the brake that in turn stops the car. Most cars have front disc brakes and rear drums, however, there are some cars, usually older ones, that have four drum brakes.

When you depress the brake pedal it forces a piston into the fluid reservoir in the master cylinder (the heart of the brake system). The fluid that is displaced travels down the brake line to the wheel cylinder, which is a double headed piston at the top of the wheel and is a part of the brake assembly. The brake assembly consists of two brake shoes, the wheel cylinder and a series of strong springs. The shoes are metal crescent shaped pieces covered on the outside with a composite of hard rubber or ceramic and metal that is designed to catch on the smooth metal of the drum. The springs hold the shoes together. The whole assembly is surrounded by the brake drum, a smooth metal bowl, and everything fits neatly on the axle, which is hidden by the tires and wheels.

When the wheel cylinder is activated, the piston is forced outward. The heads of the piston connect with the brake shoes, forcing them outward, expanding to make contact with the inside of the drum, thus stopping the car.

When the brake is released, the fluid is sucked back into the master cylinder and out of the wheel cylinder. This releases the pressure on the piston and the brake shoes return to normal via the spring action, thus allowing the wheels to turn freely.

As you can see the brakes would be totally useless if a leak should develop anywhere in the system. A small leak would be detectable to you, the driver, as a loss of braking efficiency.

A squealing noise could mean that the shoes are worn. When the composite material wears away sufficiently to expose the metal of the crescent then that metal rubs against the metal of the drum and causes a squeal. It is best not to let this go on for too long as it can cause major damage to your brake system and make your brake drums grooved. A trained mechanic can replace the shoes and resurface (turn) the drums for you. They will also tell you what else is needed to get you back on the road. Of course, someone who knows what they are doing should only attempt these repairs.

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