Drum Brakes: The Beat Goes On

[Counterman]

Nineteen sixty-five was a unique year for the Chevrolet Corvette. It was the first year for disc brakes, and they came on all four wheels. The odd part about it was you still could order four-wheel drum brakes as an option, and receive a substitution credit since they were less expensive than disc brakes. Most people were ecstatic about the change, but there were some who stuck with old faithful: the drum brake.

Drum brakes work very well – for many reasons – but there’s one overriding factor that led to the popularity of disc brakes: heat dissipation. All brakes are nothing more than a way to convert mechanical energy into heat energy. When the brakes are applied, the friction of the pads or shoes slows the vehicle down and generates heat in the rotor or drum as that mechanical energy is transferred to heat energy through friction.

The challenge with all braking is what to do with that heat. When too much heat builds up in either the fluid, the friction material (pads or shoes) or the rotors or drums, there’s a loss of braking force, which is known as brake fade. The goal with all braking systems is to remove the heat. The more heat that’s removed, the more you can put back in. Disc brakes do this very well – especially those with vented rotors. They have a constant airflow passing over them and through them, allowing them to dissipate heat rapidly.

Heat retention is the only real drawback to drum brakes. Even though most of the larger drums have cooling fins cast into them, it’s still inherent to their design that the heat is trapped inside the drum and simply takes longer to dissipate. Early on, no one had problems with drum brakes. Over time, as cars got faster and auto racing became popular, the weak point of drum brakes became more evident.

But I still like drum brakes, and one of my favorite historical points of interest is that prior to 1965, the top (drum) brake option for the Corvette included heavy-duty metallic linings, special drums and forced-draft ventilation. The design and ultimate use of the disc-brake equipment required considerable research and engineering because the performance drum brakes worked so well that it was questioned whether they could match that performance with a disc brake. It’s for this reason that the very first disc-brake design on the Corvette featured four-piston calipers at all four wheels – a design that would outperform the current drum brakes.

Drum brakes still are commonly used as rear brakes, especially on trucks. It’s true that drum brakes are less expensive – and some will cite that as one of the main reasons – but that’s really only a small factor related to their use.

Advantages of Drum Brakes

Operationally, drum brakes have many advantages over their disc-brake counterparts. One is that they are self-energizing. What this means is that as the brakes are applied, rotation of the drum will draw the shoes into it, effectively applying additional pressure to the brakes without additional effort from the driver.

If you’ve ever driven an old vehicle with four-wheel drum brakes – even one without power assist – they stop very well without a great amount of effort. The self-energizing feature of drum brakes contributes to this, but it’s important to note that the self-energizing feature of drum brakes makes brake modulation difficult, which is another reason that disc brakes are preferred for performance driving. Brake modulation is the ability to precisely control the desired amount of braking force.

Have you ever wondered why drum brakes seem to last so long? Quite often when you perform an inspection of drum brakes, they’re still in good condition, even when you’ve already replaced the front brakes on the same vehicle two or even three times. This is because the surface area of the shoes is much greater than that of disc pads, so the brakes can perform the same amount of work with less effort. Even though front brakes are responsible for the majority of braking, if you have two comparable vehicles – one with four-wheel disc and one with front disc and rear drum – you still will replace rear disc brakes at least twice, if not more, compared to how often the drum brakes would need service.

Another advantage of drum brakes on the rear is the parking brake. The levers and mechanisms that make the parking brake work are of basic mechanical design, and are easily and inexpensively incorporated into the drum brake. In addition, the self-energizing property of drum brakes not only works in either direction, but also adds to their gripping power, so the parking brake is very effective for heavier or loaded vehicles, as well as forward or backward (meaning parking on a hill is no problem).

The effective nature of a drum brake as a parking brake is why most trucks that have four-wheel disc brakes have a small brake drum machined in the center of the rear rotors, and a complete set of drum brakes (cable-operated only) that are there for the sole purpose of a parking brake. Oh yeah, and the 1965 Corvette had the same setup for parking brakes.

How They Work

Now that we’ve gone over the basic theory and some of the pros and cons, let’s look at how a typical drum brake works. I use the term “typical” here, because there are many different functional drum-brake designs. But overall, the theory behind them is the same.

The brake shoes are mounted onto a backing plate, held in place by springs that allow them to move and pivot as required during use. The shoes rest against the backing plate on multiple contact points. In between the shoes, usually located at the top, is the hydraulic actuator, referred to as a wheel cylinder. There also is a brake adjuster between the shoes, and a number of springs that aid the return of the shoes to their normal rest position after braking.

Most drum brakes also contain a self-adjusting mechanism. As the shoes wear, it keeps them adjusted close to the drum, so they contact it right away under braking. Drum-brake shoes need spring assist to return during non-braking, so they don’t continue to self-energize. For these reasons (unlike disc brakes, which are self-adjusting by design), drum brakes must be adjusted on a regular basis. They’ve always had adjusters, but it was a manual process and a normal maintenance requirement. The self-adjusting mechanism eliminated regular maintenance.

When the brake pedal is depressed, brake fluid is forced into the wheel cylinder, and the pistons in the wheel cylinder are then forced out, applying pressure to the brake shoes. The shoes are pushed into the drums and the self-energizing effect takes place, increasing the braking force. When the pedal is released, the springs between the shoes draw them back in and the fluid returns to the master cylinder.

Service Life

What makes drum brakes wear out? Naturally, the brake shoes can wear out, and the drums as well. All brake drums have a wear limit for the inner diameter. In many cases, the drums can be resurfaced on a brake lathe. However, if they’re outside of their wear limit, they must be replaced.

Since the shoes typically last a long time due to their surface area, the lining itself is often OK. Some of the most common problems include leaking or seized wheel cylinders; rusted or broken springs and hardware; leaking axle seals, which allows differential oil to contaminate the brake linings; and seized parking-brake mechanisms and cables. Rust also can take its toll on the backing plates as well.

Some brake shoes are made with the lining riveted to the shoe; on others, the lining is bonded to the shoe. In some cases – primarily related to age – the bonding will begin to fail, and the lining will separate from the shoe.

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When it comes to service, it’s not uncommon to find drum brakes that are in good condition, but the wheel cylinders have begun to seep fluid. It’s acceptable in these cases to replace only the wheel cylinder (as long as fluid has not gotten on the linings), clean the hardware and lubricate the brake-shoe contact points.

On the flip side, if the shoes are worn out or contaminated or if hardware is rusty and old, it only makes sense to replace shoes, hardware and wheel cylinders at the same time. Even if the wheel cylinder isn’t leaking, if you don’t know its age, you’re better off replacing it. It’s not worth risking a leak a short time down the road. 

When replacing brake shoes, a common practice is to do one side at a time, so you always have one side assembled for comparison. This is always a good idea. Even if you think you’ll remember where everything goes, when you start to put drum brakes back together, the pile of springs and hardware can begin to look more like a Rubik’s cube than anything else.

One of the most important details is the cleaning and preparation of the backing plates. Commonly overlooked are the contact points between the shoe and the backing plate. These often get grooved where the shoe rests, and/or rust builds up around the spot, creating the same affect. These contact points should be cleaned or sanded until they’re smooth, so the action of the brake shoe isn’t restricted. If the contact points cannot be smoothed out or if the backing plate is rusty and disintegrating, it should be replaced.

A DIFM Shopping List

While the standard brake-drum repair includes shoes, drums, wheel cylinders and hardware kits, there’s a lot more that you can recommend. Brake fluid, of course, is on the list, but here’s a complete list of items to turn it into a professional job:

1. Standard hardware kits. Standard hardware kits include return springs; hold-down springs, pins and cups; and adjustment window plugs. There usually are a few extra parts since the kits are designed to fit multiple different applications. It’s important to note that the standard kits don’t include specific parking-brake adjusters or hardware.

2. Parking-brake adjusters and hardware. Parking-brake adjusters and hardware are not part of most hardware kits, but they are an essential part of brake operation. Even if the original adjusters look OK, close examination usually will show that there’s enough wear on the self-adjusting mechanisms to prevent them from working properly.

3. Brake lubricant. It can handle the heat of brakes, it’s designed to stay in place and not wash away and it’s designed not to damage any of the rubber seals and components it comes in contact with. Use it sparingly on the contact points of the shoe to backing plate and brake-shoe pivot points.

4. Parking-brake cables. If there’s any question about cable condition, this is the time to replace them. They should operate smoothly and freely.

5. Backing plates. Often ignored but readily available, if backing plates are severely rusted or grooved deeply where the shoes rest, they should be replaced. Once the brakes are disassembled, it’s usually not much extra work.

6. Since the majority of drum brakes are on the back of trucks or vans, it’s not uncommon to have an axle-seal leak. If this gear oil contacts the brake linings, it will ruin them. At minimum, axle seals will need to be replaced, and sometimes there are worn bearings or axles that are the culprit as well. This opens a lot of doors for additional sales for rear-axle service.

7. Special tools. You may be able to get by without, but there are several special tools for drum brakes that make the job go much easier. Hold-down and return-spring tools save a lot of time, and brake-adjusting tools also are very useful. Final brake adjustment always is performed with the drum on, and brake-adjusting “spoons” work a lot better than screwdrivers.

8. Brake/parts cleaner. A must for drum-brake jobs.

Tips for DIYers

A do-it-yourselfer might have a lot of questions about drum brakes. Replacing drum brakes generally isn’t hard, but it’s important to take your time. Here are a few pointers that could help your DIY customers get the job done right:

1. Primary vs secondary shoes. When you look at a set of brake shoes, you’ll see that the linings are different lengths. These are referred to as the primary and secondary shoes. During braking, the rotation of the drum moving in a forward direction will draw the front (primary) shoe into the drum. That motion then is transferred into the rear (secondary) shoe. This, again, is the self-energizing effect. Because it initiates with the front shoe, the front shoe provides a greater braking force. So, in order to balance the force of the two shoes, the rear shoe has a greater surface area of lining. I will cautiously say this is always true. But, this highlights the importance of doing one side at a time. As mentioned before, there are many different functional drum-brake designs, and it’s possible that there’s an application where this could differ. So, always advise on the side of what you know is true the majority of the time. But, if someone is working on an oddball application, make sure they research it.

2. The balance of the brake drum is very important. Just like a wheel that’s out of balance, a brake drum can cause severe vibration for the same reason. Most of them have weights welded on the outside when they’re balanced during production. These weights can interfere with some aftermarket wheels. If that’s the case, the drums will have to be balanced in a different manner. Don’t just grind away or break off the weight.

3. You’ll see a lot of vehicles with rear drum brakes, but very few with front. They’re out there, however, and they will appear from time to time. On vehicles with four-wheel drum brakes, the front brakes are larger in size from the drums to the shoes, so the parts will be different.

4. If drums brakes aren’t properly adjusted, it will result in a low brake pedal and uneven wear, and the vehicle can pull to one side during braking. When it comes to adjusting them, the best procedure is to adjust them by hand until they’re close to the drum, but so you can still slide the drum on and off easily. Once you reach that point, with the drum installed, seat the brakes by depressing the brake pedal multiple times. If wheel cylinders have been replaced, they’ll need bled at this point. The brake pedal should feel good if the initial adjustment is close. Finish up the adjustment with the drum installed, using a brake-adjusting spoon through one of the access holes in the backing plate or on the drum itself. Adjust the shoes outward until the drum begins to drag moderately, then back off the adjuster until the wheel spins freely. This usually takes three to four clicks of the adjuster. Very slight drag on the drum is acceptable. Experience is the best teacher.

5. Last but not least, clean, clean, clean, and prepare the backing plates so the shoe contact points are smooth. Replace the backing plates if necessary, and all hardware.

Armed with these tips, you should be able to get your customers everything they need for successful drum-brake replacement and answer their questions. So, all that remains is what do you want on your classic Corvette?

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