5 Common Problems With Drum Brakes

Brakes are arguably the most important single system on any vehicle, perhaps along with tires. After all, it doesn’t matter how powerful your engine is or how well your car is planted — if you can’t stop, you’re no longer in a car, you’re in a missile.

Brake systems are fairly simple and intuitive if you understand how they work. In most cars, when you press on the brake pedal, it acts on the brake master cylinder. This applies pressure to your brake fluid, which is incompressible. This brake fluid then acts upon the brakes themselves, squeezing them against the working surfaces and using friction to slow the car down. It’s not the only braking method — systems like Brembo’s non-hydraulic Sensify are out there in various stages of development, but the hydraulic system is by far the most common.

On the other end of the scale from Sensify, a drum brake appears quite complicated at a glance — when you open it up, there’s a bunch of springs, a cylinder, and inner surfaces. Let’s break it down.

Imagine a spinning bowl. You put your hands on the inside of the bowl — your hands are the brake shoes. You push outward and use your hands to slow the bowl down — that’s what the wheel cylinder does. A drum brake is just two big half-moon shoes being forced up against a bowl. Likewise, the parking brake has a cable that pushes the shoes outward through its own actuator, acting like a secondary brake pedal. Now that we have a basic understanding, here’s what can go wrong.

You might’ve come across this term a few times, even in more modern disc brake-equipped vehicles, and that’s because of how brakes work in the first place. Brakes convert kinetic energy to heat — pushing on that bowl creates a lot of friction, and that quickly adds up like rug burn. Brake fade is essentially the braking equivalent of rug burn; it means your brakes are overheating and subsequently lose stopping power.

Brake shoes use friction material to slow the car down through mechanically rubbing up against the shoe, similar in principle to how a clutch in a manual transmission works. When you ride the clutch in a manual transmission, you’re building up a lot of heat because the clutch and engine are spinning at different speeds, so it’s constantly giving that friction material rug burn. If it can’t accept any more heat, it can’t convert any more kinetic energy and it stops working. Similarly, if you ride the brakes without actually stopping the car — if you’re on a steep hill, for example — the same phenomenon occurs.

Brake fade is more common in drum brakes for two reasons — disc brakes are more exposed to ambient rushing air than the inside of drums, and drums expand when superheated. That expansion means that the shoes can’t reach the sides as well, so your brake pedal starts to feel spongy and unresponsive. The easiest solution to brake fade is to simply stop the car for a while and let them cool. Moreover, a good way to prevent it in the first place is to practice engine braking to avoid overstressing your brakes on steep hills.

Brake friction material is a finite resource on your car. Every time you apply the brakes, a little bit of that friction material wears off, usually turning to dust — that’s why the inside of your brake drum can look like a charcoal briquette after years of driving. That also means that, eventually, this friction material will wear to nothing, at which point you’ll lose most of your car’s braking performance.

Wear occurs almost constantly; it’s a necessary evil, but that’s the nature of the game. Luckily there are a few ways of preventing brake wear, and most of them are common sense. For instance, don’t ride the brakes when you’re cruising along, otherwise you’re just building up heat for no reason. Don’t apply too much pressure by panic-braking all the time; focus on smooth driving. Don’t carry a ton of weight all the time, because that leads to the brakes having to work harder. Of course, the same also applies to disc brakes, but remember that drums have a harder time dissipating heat.

Regardless of how quickly your brake shoes wear, you’ll often immediately notice if they’re worn when your brakes start squealing. But squealing brakes aren’t necessarily the end of the road — many shoes actually have little pieces of metal embedded in the friction material. These acoustic indicators act as a warning to let you know your brake shoes are on their way out and should be replaced; it doesn’t necessarily mean they’re gone entirely. Other brake shoes might have electronic warnings to let you know the same thing, but in any case, if you hear and/or see such a warning, it means it’s time to change your brake shoes.

How do you know if your brakes need adjusting? If you open up a brake drum, you’ll see a complicated arrangement of springs, levers, and a big cylinder on one side. The way all this works is that the cylinder pushes on the top of both brake shoes, while the springs themselves do two things: The first is set how far apart the shoes are at rest, and the second is to ensure the top and bottom of the shoes move in unison. Basically, if you push on just one side of the shoes and not the other, you’ll only engage the top and never use the friction material on the bottom. Such uneven wear is called wedge-wear. Time for adjustment.

Another factor to consider is how far down your brake pedal travels before you feel anything. Drum brakes push outward against the surface via the cylinder, but they also have a resting position that’s determined by all the hardware within the brake — those springs and levers. As the brake shoe wears down, it’ll be physically further from the drum, meaning you’ll need to periodically adjust it to keep the brakes feeling tight.

Likewise, the reverse can be true as well; you might have brake shoes that are too tight and are constantly wearing on the drum. In this case, you’ll likely smell burning after a time as the shoes glaze the inside of the brake drum, overheating it and polishing the friction material with a glass-like smoothness. In either case, it means it’s time to change some components and adjust everything properly.

Here’s another problem common to both drum brakes and disc brakes; at the heart of both systems lies a series of lines filled with hydraulic fluid. There are many different variants of brake fluid, granted, but they all serve the same function: They are incompressible liquids designed to resist the force placed upon them by the brake master cylinder. The problem here doesn’t arrive with the brakes themselves — rather, it’s to do with the brake lines.

Under ideal conditions, your brake system is airtight, and you want it that way because, unlike brake fluid, air can be compressed. When you press on a pedal filled with air, the best-case scenario is the pedal will have an unusually long travel before engaging; you might only have smaller bubbles in the system, but generally what’ll happen is you press on the brake pedal and it goes straight to the floor. Nothing happens, or very little happens right at the end of travel. This means you either sprung a leak or you have bubbles in the system from bleeding the brakes poorly.

Luckily, assuming you haven’t burst a line and your master cylinder is alright, this is a pretty straightforward fix. Hydraulic lines interact with the drum cylinder, forcing it to expand and pushing on the shoe, so you disconnect the line at that side and bleed the brakes like any disc brake. You can even do this job yourself in an afternoon using nothing more than a hose connected to a small bottle to catch any fluid.

We’ve already discussed one issue you might’ve noticed, which is that high-pitched squealing you get when your brake pads are too low. This is a more advanced stage of that same phenomenon, occurring when you have no usable friction material left and it’s effectively just metal on metal contact. It could also be caused by contaminants like dirt and metal particulates, particularly relevant if your vehicle goes off-road or travels to job sites frequently — bear in mind that a lot of trucks today still have drum brakes.

If you know anything about trains, you’ll know right away why this is a problem. Trains run on steel rails with steel wheels because of its extremely low friction coefficient; that’s why trains are so good on fuel, as opposed to trucks. The problem comes when you apply that principle to brakes. Imagine for a second what it’s like trying to stop something like a loaded car traveling downhill with nothing more than two pieces of steel friction-welding each other together.

Of course, that’s a bit extreme, but it gets the point across. A huge variety of contaminants can affect your braking performance in this manner, especially factors like rust and corrosion. Rust affects different components in different ways, ranging from seizing the drum cylinder on to freezing your emergency brake in-place. Even your brake shoes can rust, depending on what friction material you’re using, which is one reason why it’s so important to drive long-term stored cars every now and then.

We selected these phenomena, first and foremost, because of their relative prevalence. Factors such as perishable materials in brake friction material, the necessity of periodic adjustment, and bleeding brake lines are common to most production cars with drum brakes on today’s roads.

The second criteria for selection was to focus solely on issues either exclusive to drum brakes, or shared between drum and disc brakes. Both braking systems feature a lot of overlap, owing to their similar functions, so crossover is virtually unavoidable; still, it’s important to acknowledge that problems shared between both systems can be equally as important and commonplace.

Our sources for this article outside of in-house expertise in the automotive field include independent automotive websites such as professional auto care centers, as well as scientific analyses and case studies of various brake drum failures to determine their root causes. We took the most common among these, along with testimonials from around the Internet, to come up with the initial list; this was then followed up with the aforementioned sources for the actual information presented herein.

This list assumes that the drum brakes in question aren’t damaged or warped by impacts or overt abuse, which constitute their own failures and were therefore not considered.