Brake Drum Service and replacement
The mention of drum brake service doesnít usually get much attention in regards to interest these days. The service is thought to be so straightforward and simple that it doesnít take much brain power to get it done. Right? Not necessarily so. Proper drum brake service, like anything else, requires attention to detail in order to provide the best possible braking.
The first step is determining the level of service needed. This involves a thorough inspection. While most shops will check the obvious, many miss the not so obvious. Take for instance the wheel cylinders. It is critical to determine if the wheel cylinder pistons move freely in their bores. A common practice is to "rock" the shoes back and forth to accomplish this check. This process will not move the wheel cylinders in a duo-servo drum brake system. The shoe assembly simply pivots around the anchor pin. The only accurate method to determine if the wheel cylinder pistons move freely is to physically move each piston using a screwdriver or similar tool. Many a vehicle has been serviced with one or more seized wheel cylinder pistons due to not checking the wheel cylinders properly.
In addition to checking the wheel cylinders for free movement, they should be checked for signs of leaking. If leaking is not visible on the outside of the wheel cylinder, a check inside the dust boot is in order. Most wheel cylinders use a soft dust boot that allows easy checking. Simply pull the dust boot back and inspect each end of the wheel cylinder. When dealing with either the press-on or press-in style dust boots, it is best not to try and remove the dust boot for inspection. On these designs, use a dull pick or small screwdriver to pull the boot back from around the bottom of the pushrod.
When checking for leaking it is important to understand what you are looking at when checking for leaks.
What defines a leaking wheel cylinder? One of the first things to understand is it is normal to see some dampness inside a wheel cylinder. This usually shows up as a light sheen on the cylinder bore or piston. This is normal. While the industry has agreed that a "leaking" wheel cylinder is reason to require replacement, nobody has stepped up to the plate and defined what "leaking" means. I use the approach that if I get one drop or more when I pull the dust boot back, it is considered a leaking wheel cylinder
(See Figure 1).
There are times when you check a wheel cylinder and there is a build up, but it doesnít drip (See Figure 2). I call this puddling and will suggest the wheel cylinder as the cause when I find this condition. A common misunderstanding about finding either of these conditions is concluding what, if any effect, it will have on the brake pedal. These leaks are non-pressure leaks and will not impact pedal height.
They occur on the release cycle when the cup seals in the wheel cylinder are not being held tightly against the cylinder wall by the pressure in the brake system. If ignored, they could turn into pressure leaks commonly referred to as a "blown out" wheel cylinder. The main point to understand is that if the vehicle being inspected has a lower-than-normal pedal, you need to look further for the cause.
If you work anywhere there is rust, you have to make sure to check the self-adjusting mechanism for proper operation. One of the most common failures is the self adjuster. Either the threads seize, the end cap seizes, or in some cases both. When the adjuster stops rotating, it is possible for the teeth nearest the adjusting lever to be worn completely off, as in Figure 3. Failure to check for this can result in reusing the part. When the adjuster rotates to the point where the teeth are missing, self adjustment will stop.
There are a couple common misconceptions about self-adjusting mechanisms. One is that they donít work. This is usually not true unless some part in the self-adjusting system has stopped working. It is easy to check how well the self-adjusting mechanism is working. Using a quick set, adjust it to the drumís widest point. Hold the quick set over the shoes at the widest point. Check to see how much gap there is between the quick set and the shoe. A properly working self-adjusting mechanism will have little or no gap. When offering a "Clean and adjust" this is the best method to use to determine what adjustment is necessary. If there is a noticeable gap, something is wrong with the self-adjusting mechanism. Manually adjusting the brakes will only have a temporary effect. You need to find out what isnít working and fix it. This process works as long as you didnít have to back the shoes off to get the drum off.
THOSE LOUSY SELF ADJUSTERSAnother common misconception is when self adjustment takes place. A popular belief is that the most common method of self adjustment is through the use of the parking brake. Of the three methods, this is the least commonly used. Basically, all duo-servos self adjust when the brakes are applied and released when backing up. The majority of non-servo drum brakes self adjust when braking when the vehicle is moving in the forward direction. Only a few systems use the parking brake for self adjusting. All the drum brakes shown in Figure 4 self adjust during forward braking.
A very effective method of applying this information in the field is to use it as part of the adjustment process. On designs with screw-type self adjusters, use a quick set to rough the brake shoes to the drums. Finish the process by doing whatever activates the self-adjusting mechanism. On duo-servos, make about six stops when backing up, making sure to fully apply and release the pedal. On most non-servos, just take the vehicle for a test drive. On non-servo systems that use a toothed cam or similar device, all that is necessary is to install the drums and drive the vehicle. The self adjusters will do the rest.
SHOE AND DRUM RELATIONSHIPA key part to successful drum brake service is making sure the brake shoes end up in the proper relationship to the friction surface of the drums. The part responsible for this is the backing plate. More specifically, the shoe lands, or bosses, on the backing plate
(See Figure 5).
These six raised areas determine if the friction surface of the shoes will be parallel to the friction surface of the drum. They are a wear part and, in many cases, do not last forever. Yet, they are often over looked during the inspection process.
Duo-servo drum brakes are harder on the bosses than non-servo. The lower bosses on a duo servo will usually be the ones to exhibit the most wear. To inspect, simply pull out on the adjuster while using a light to check the condition of the bosses, as in Figure 6. It is not uncommon to find bosses that look like Figure 7 on a regular basis. These grooves will change the position of the brake shoe in relation to the drumís friction surface.
This could cause uneven wear and an over-sensitive brake. The grooves can also cause the brake shoe to hang up on apply or release, resulting in erratic braking.
The question, once you find this condition, is what to do about it. Of course, one option is to replace the backing plate. This is a perfectly acceptable route to take, but there is another option if the rest of the backing plate is in good condition.
That option would be to repair it. In many cases repairing the backing plate can be done faster than replacing it and can generate as much profit as replacing it. If the choice is made to repair it, the most important part of the process is to restore the shoe land to the proper height. All the shoe lands must lie in the same plane. When repairing the shoe land, you must restore it to its original height or the repair will do more harm than good.
The repair process is straightforward. The first step is to clean the shoe lands. When cleaning any shoe land, it is critical not to remove good material. Use an angle grinder or abrasive blaster to clean the surface, as in Figure 8. The next step involves filling the wear spot. A mig welder is the best method to accomplish this. You want to avoid over filling the groove or it will make the next step that much harder. The last step involves grinding the weld down to restore the shoe land height. Generally, you want to repair one shoe land at a time. The reason for this is you will use the shoe lands nearest the one being repaired for reference surfaces. Grind the weld down to a point where you think it is close. Try not to grind it to far or you might have to add more weld.
When you get it close, you need to check its height. I make a template from a muffler shield bracket by bending it to the same arc as the brake shoe. Hold it against the shoe land being repaired and the shoe lands closest to it, as in Figure 9. Check to see if it rests flat against the three surfaces. If not, remove some more material. Repeat this process until the template rests square against all three shoe lands. Repeat this process with any other shoe land in need of repair. Before installing the shoes, make sure to lube all the shoe lands with a high-quality moly lube. Remember, the shoe lands are also the friction surface for the shoes, so lubing is an important part of the process.
READ THE SIGNS
wear pattern of the rear shoes will tell you a lot about how the
shoes are contacting the drumís friction surface. Take for example
the primary shoe shown in Figure 10.
Looking at the top outer edge of the friction surface, it is obvious that it is not contacting the drum.
Looking at the inner-to-outer edge thickness shows a taper wear condition. This would indicate either a tapered drum or the shoe cocking during application.
A look at the secondary shoe shows even wear which eliminates the drum. A close look at the shoe-to-shoe land fit on the bottom of the primary shoe determines the cause.
In this drum brake, the primary shoe has worn in the shape of the shoe land allowing the shoe to cock. This is the cause of the misalignment evident on the shoeís friction surface.
This misalignment is causing another problem. The edge of the secondary shoe shows signs of it rubbing the inside of the drum, as seen in Figure 11. This is also being caused by the worn primary shoe.
The duo-servo braking action forces the primary shoe to ride up the edge of the shoe land. This causes the adjuster to push out on the bottom of the secondary shoe, causing it to rub the drum.
The wear condition of the primary shoe is not common, but points out why it is so important to do a thorough inspection.
TO LUBE OR NOT TO LUBE?Another important aspect of performing drum brake service is determining what points in the drum brake assembly require cleaning and lubing. What you donít want to do is spend time doing what I call "cosmetic cleaning." This is cleaning that will have no positive impact on the operation of the brakes. A typical GM duo-servo drum brake is shown in Figure 12. The lube points are identified by the arrows.
In addition to the shoe lands, the other clean and lube points include the self adjuster and the pivot point of the adjusting arm. Figure 13 shows an important cleaning point in some non-servo drum brakes. The adjusting teeth on the cam and peg can get filled with rust and corrosion. This can prevent the teeth from properly meshing and prevent self adjustment. Cleaning and lubing the parts correctly willensure a quality, long lasting brake job.
Grease and brake fluid are among the worst things to get on brake shoes during the installation process. To avoid this common pitfall, you have two choices. You could go wash your hands before installing the new shoes or you could use a pretty neat little trick. Looking at Figure 14, you can see what that trick is. Before handling the shoes, put some low-tack 3-inch masking tape over the friction surfaces. You would be hard pressed to contaminate these shoes during the service process. When done, just peel the tape off and there you go.
Recreational boating and jet skiing are common past times for an ever-growing population. The rear drum assemblies on the vehicles used to tow these watercraft are constantly going in and out of the water. This constant exposure to moisture can dramatically shorten the life of the wheel cylinders, even if the dust boots are in good condition. Moisture will seep in and around the inner and outer seal of the dust boot causing the bore and piston to corrode. A simple step during the installation of the shoes or wheel cylinders will help form a tough moisture barrier.
Using a high-quality silicone paste, put a thin layer on the inside of each dust boot and around the inner and outer sealing surfaces. When treating the sealing surfaces, put the silicone on both the boot and wheel cylinder housing and pushrod, as shown in Figure 15. When the two surfaces come together they will form a virtually impenetrable moisture barrier. Make sure to tell your customer so you get the credit you deserve.
MAKING MORE WORK FOR YOURSELF?
A common service offered, both as part of front brake service and a stand alone profit generator, is the old "Clean and Adjust." A couple of important points should be made here. The first involves the cleaning process. Many shops use brake clean to perform this, not realizing what they might be doing. We all know that we techs often subscribe to the saying "If a little is good, a lot is better." This is typically the approach taken when performing the cleaning. This is good news for the brake clean manufacturers, but bad for the drum brakes. What you have to consider is that if the drum brake assembly had any lubrication on the bosses, pivot points and self adjuster, it may not anymore! The cleaning process needs to be taken back a step. Proper drum removal involves the use of a birdbath to control the dust. This device should also be used to perform the cleaning. It is water based and will do a fine job of removing the dust that has accumulated.
Next comes the adjust. The typical approach is accessing the adjuster through the backing plate hole and cranking away. There is a much simpler way, but it may require an additional step. Many drums develop a noticeable rust ridge. If the adjusters are expanded inside this rust ridge, I wouldnít want to be the next guy who has to take those drums off. The problem is, if you do your job right, you are going to be the next guy!
A better way to approach this would be to machine the rust ridge off using the lathe. This will allow you to use the procedure we discussed earlier using the quick set and finishing up with the self-adjusting system. The other advantage is that whoever has to remove the drums the next time will thank you.
LOW PEDAL, CRANK ĎEM UP
One of the most common "fixes" applied to excessive pedal travel is to adjust the rear brakes. While this may improve pedal height, it may also hide the real culprit. Rear brake adjustment should only be performed when a line lock test points you to the rear brakes. If the pedal is good before you pull the rear line lock off, and pulling the rear line lock off duplicates the excessive travel, you are on the right track. If this happens, it is either an adjustment problem or air from the line lock down to the wheel cylinders. If you determine it is the rear adjustment, donít just adjust them out without determining why they were not self adjusting first.
REPLACING REAR BRAKE HARDWARE
Some shops religiously replace rear brake hardware with each set of shoes, while others only replace it if something fails. The industry currently says replacement is required only if something is bent, broken, missing or distorted. Generally, it is this authorís opinion that hardware should be suggested with each set of shoes installed. An important aspect regarding the function of the return springs is often not factored in when making this decision. The common belief is that return springs have only one job Ė to return or release the brake shoes after the pedal has been released. While this is an important job, it is not the only one.
The return springs also determine at what point the rear brakes will begin to apply. The weaker the spring, the sooner the apply. Rear brake shoes last considerably longer than disc brake pads on most vehicles. A typical rear-wheel drive will wear two sets of pads to one set of shoes, while a FWD might go four sets of pads before the shoes need replacing. Some vehicles could have in excess of 80,000 miles on them before the rear shoes need replaced. After this many miles the springs have stretched and relaxed, and heated and cooled hundreds of thousands of times. They cannot help but loose some of their tension. The loss of tension changes the timing of the rear brake apply allowing them to apply too soon. This results in an overly sensitive rear brake and can lead to problems. Rear brake hardware is a small price to pay to restore such an important part of rear brake operation.
Details are one of the things that sets one shop or technician apart from the next. In the area of drum brake service, it can make or break the quality of the job. Rear drum brakes will probably be around as long as hydraulic brakes, so making sure you are on top of what their service involves is an important aspect of providing quality service to your customers.
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