How to Tell if You Have Bad Brake Drums

Mia Bevacqua
August 3, 2018

The safety of your vehicle depends on the brakes. Worn or damaged drums should be addressed as soon as possible. If your car is having trouble braking, stop driving it and have it towed to a repair shop

Here are some signs your brake drums are going bad.

Noise: When the friction material wears off the brake shoe, its metal backing plate will begin to dig into the brake drum. This will cause a metallic scraping or grinding noise.

Low brake pedal: A low brake pedal can result from excessive clearance between the shoe and drum. This is commonly corrected by cleaning and adjusting the brakes, but could indicate a brake fluid leak or bad master cylinder.

Spongy brake pedal: Drums that are cracked or worn thin can cause the brake pedal to feel spongy when you press it. However, spongy brakes are most commonly caused by air in the brake lines.

Brake pedal pulsation: Brake drums that are out of round do not make constant contact with the shoes. Instead, the shoes bounce up and down causing a pulsation in the brake pedal.

Parking brake doesn’t hold: In some extreme cases, if the drums are worn thin enough, they may not make contact with the shoes. As a result, the parking brake will not hold. Signs of a parking brake trouble include a loud pop when putting the vehicle in gear after parking on a slope (if you have an automatic transmission) or a parking brake lever or pedal that is way too easy to set or doesn't return once released.

» LEARN MORE: Get an estimate for your brake drum replacement

How to fix the problem

Before spending time and money to replace the brake drums, a thorough inspection should be performed. Drums that are too thin will crack and break when they get too hot or the material will glaze, which reduces friction.

In some cases, it may be possible to resurface a problematic drum — or having the brakes cleaned and adjusted might do the trick. 

Measuring the inner diameter of the drum and comparing it to the manufacturer’s specification will determine whether is t can be reused. A drum that is too worn out will need to be replaced.

Brake shoes should always be replaced whenever the drums are swapped out or resurfaced, so that they share the same wear pattern. Because the shoes are held in place by a collection of small springs, replacement can be tedious and is best left to a professional.

Get it diagnosed by a professional

How brake drums work

Brake drums and shoes used to be on pretty much every car, but these days, they’re typically only found on the rear wheels — and even that’s a rarity. In fact, the vast majority of new cars have disc brakes on all four wheels.

But many existing vehicles do still have brake drums, and even those with all disc brakes tend to have a smaller drum-and-shoe arrangement for the emergency brake.

Brake drums are circular metal components that house the brake shoes. During braking, the shoes press against the drum to create friction needed to stop the vehicle. 

To better understand the role of the drums, it's helpful to have an overview of how the drum brake system works.

The heart of any braking system is the master cylinder. It takes the force the driver applies to the brake pedal and converts it into hydraulic pressure. When the brakes are applied, pressurized fluid flows from the master cylinder, through the brake lines, to the wheel cylinders, which sit between the brake shoes. Fluid pressure causes the wheel cylinders to force the shoes outward against the inner part of the drum. This creates friction that brings the vehicle to a halt. 

Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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