Stopping your car is almost as important as making it go, and whether you’re a left- or right-foot braker, it’s safe to say you want to make sure your car’s clampers are up to the task. For our next installment of Hooniverse 101, we’re going to dive into the function and maintenance of those most mysterious of brake types – the drum brake.
Now, as a reminder, Hooniverse 101 is intended as an introduction to auto maintenance and function, so many of you will be ahead of the curve when it comes to much of what we’ll discuss. Your input on the discussion, in the comments below, is well appreciated. After all, this is for those of us who don’t know how their vehicles work, but are eager to learn the basics.
When I did the last Hooniverse 101, on oil changes, I got a lot of feedback that Drum Brakes would make for a valued topic, so here we are. It should be noted however that due to the number of minor differences between various makes’ and models’ drum brake designs, you shouldn’t tackle this job without a good manual or detailed instruction for your own particular car. This however, will hopefully give you a good preparation for what you could expect.
Spin the Drum Slowly
Drum brakes have been around for seemingly forever, and were the stoppers of choice for most of the last century. They supplanted externally clamping brakes – literally a friction material-lined metal band that through mechanical means would squeeze either an axle or a wheel on the car’s driveshaft.
Needless to say, as cars became more capable at traveling at speed, the need arose to safely retard that capability, and in the 1930s the industry made a shift from mechanical brakes to hydraulic. The braking mechanisms of today are, at their basics, riffs on these decades old designs.
But what about drum brakes? How the heck do they work? Well, pretty simply as you will see in my professional-grade illustration below, or in this lengthy video produced by the Army.
The drum brake is composed of four basic parts that stop the car, and a bunch of others that help keep it bolted together and provide other functions. We’ll get into those in a minute, but to start out let’s review the basics.
Like I said, there are four basic parts in most drum brakes that are required for them to function. Those are –
The brake drum, which is the part that rotates with the wheel.
The brake shoes, which come in pairs, are covered in a sacrificial material, and are pressed against the inside of the drum by hydraulic pressure.
The hydraulic or wheel cylinder, which converts hydraulic fluid pressure to mechanical pressure against the shoes.
The return springs, which serve to pull the brake shoes away from the drum preventing drag and heat generation.
That’s pretty much it. Now, all drum brakes are not created equal, but most of them work in this fashion- the hydraulic cylinder sits at either the top or bottom of the brake backing plate. Opposite that is a pivot block that serves as the fulcrum against which the brake shoes are pushed.
When the brake pedal is pressed, hydraulic fluid is sent under great pressure from the master cylinder to the wheel cylinder. A piston in the wheel cylinder pushes outward on either side on the ends of the brake shoes which are resting against the piston ends.
The pivot block serves to align and keep the shoes from simply being pushed through an arc by the expanding pressure. Because of these factors, the shoes come into contact with inside surface of the rotating brake drum. As the shoes rub against the drum, speed is converted to heat and brake dust, and the car is slowed by the friction.
Some things to keep in mind – there are typically additional parts inside the brake drum and poking through the backing plate. This is the parking or emergency brake, which is a cable-operated mechanical means of pushing the brake shoe against the drum lining and keeping your car from rolling down the hill.
There are also drum brakes that lack a Pivot Block, instead having a pair of wheel cylinders – one top and one bottom – that each act on a separate shoe. While they’ll look different their basic function remains the same as that of the Datsun I’m using as an example.
The most important thing to remember when changing your drum brake’s shoes or rebuilding a wheel cylinder is to work on one-side at a time and use the other as a benchmark for where all the parts go. With the number of springs, and the fact that there are two fairly similar looking shoes with which to contend, this is critical advice.
Never tackle your brakes for the first time by simply disassembling everything thinking you’ll remember the orientation of the springs, or how the hand brake levers fit. Also, take a lot of pictures as you take stuff apart so you’ll at least be able to retrace your steps photographically.
Okay, let’s go through the step-by-steps of changing out the brake shoes on this 1970 Datsun.
First thing you will need to do is loosen the lug nuts on the wheels. Do this before jacking the car up or else you’re gonna’ have a bad time. Remove any hub caps or other encumbrances to getting to the lugs and loosen them to the point where you’ll easily be able to spin them off when the wheel is elevated and free spinning.
Next, we have to jack that bad boy up. Chock the opposite end wheels and position your jack in an acceptable place under the car – either in a factory provided jacking spot, or – as in the case here – under a well braced element like the differential pumpkin. Once the wheel are off the ground and the car is at a good height for you to work on it without turning into Igor, support it by placing jack stands under substantial frame elements.
I can’t stress this more strongly: DON’T JUST LEAVE THE CAR ON THE JACK. Put it on stands like I have, or by placing wide wooden supports under the frame where they won’t interfere with your access to the wheel arches.
Drop the jack and give the car a gentle push to ensure that it is securing supported. Feel free to winch a bit as you do this because it would be scary if the car actually fell.
Once you are sure the car is safely supported, go ahead and remove the wheel nuts/studs completely and pull off the tires. Put them in a safe place, and you might want to go as far as bagging your nuts in a zip-lock to ensure they don’t get lost.
Before we go any further, a word about your safety. Many older cars have brakes with friction material made out of stuff like asbestos, which is gnarly stuff and bad for your lungs. Also, some of the chemicals with which we will be working are not so good to have come in contact with your skin. Because of these factors, you should invest in a good particulate mask and some nitrile gloves for protection.
Okay, back to the play by play.
We now have a good view of the brake drum itself. This one happens to be made of aluminum and finned, but most are cast iron and exhibit a patina of rust. You may also find that they have either a large Phillips head screw flush with the outer face near its center, or a torx head in the same spot if the car’s newish. You’ll need to back those out as they are there to hold the drum in place when the wheel is not doing so.
In this case, the drum is free floating, and only needs to be pulled off the wheel studs to be removed. Should the shoes be rubbing against the drum, or if corrosion is sticking it to the hub center, then you’ll need to take some additional steps.
If rubbing, there will typically be an access hole either in the drum face or in the backing plate where a screwdriver may be inserted and the auto adjuster (more on that later) could be backed off. If it’s corrosion, then you’ll need a liberal application of penetrating oil and a hefty rubber mallet to help break things free. Don’t hit it too hard, if it doesn’t come off with modest attention, then you may want to enlist professional help.
Also, once off you can take your drums into most parts and service facilities where, for a nominal fee, they will turn them to machine the inner surface and ensure they are not out of round.
In my case, the drums slid off without issue, giving us a view of the drum brake’s inner workings. Here you can see the shoes, the hydraulic cylinder, pivot block, and return springs. I measured the drums and found both to be within spec, so I didn’t bother with having them turned.
First off, just like Mister Rogers, we’re going to get those old shoes off. There are a pair of what are known as anti-rattle pins holding the shoes in place. These are made up of four parts – the pin, the spring, bottom seat, and cap. The pin actually looks like a nail with a flat end on it instead of a point. That flat end slots into a slot on the cap, and when spun 90 degrees holds the whole shebang together – along with the shoe – against the small spring’s pressure.
Here’s most of the tools we’ll be using to get the old shoes off and the new ones in place, and the most important of those is that pair of long nose pliers. You can use a special tool for releasing the pins, but I find that this work the best. Grab the pin with the pliers, push in and twist, and they come apart easy as pie.
Once the anti rattle pins and springs and caps have been removed, the next step is to pull the shoes off of their perch on the pivot block. This is easily accomplished with a pair of good sized channel locks, grasping the metal ahead of the friction material twisting outward. Once off of there, the shoes will typically have enough tension removed to slide off the perches on the other side, and out.
After those are out of the way, it’s time to do a little house cleaning. First thing is the brake’s self adjuster. That’s the little gear-like wheel on one side of the hydraulic cylinder. As the friction material wears, the adjuster pushes the shoes further outward in compensation. They work every time you apply the brakes when backing up and now need to be screwed back down to make room for the new, thicker shoes.
You do this on the Datsun by spinning the little cap in the center of the adjuster, it threads back down and I went ahead and just seated it all the way down, making sure the slot in its top was properly facing vertically.
Also, it’s a good idea should your car be a bit older to hit all of the components with a dash of brake cleaner to get rid of the grime. Make sure to put a catch basin under the brake to catch the drips, and don’t breathe that stuff in, it’s pretty bad for you.
I also recommend cleaning all of the bearing surfaces and the studs of any crud or surface corrosion as that will make for a better reassembly and eventual function. I hit everything with a rotary wire brush and it cleaned up pretty nicely.
This is also the time to check the wheel cylinder for any tears in its rubber boot or apparent leaks. Make sure that while the shoes are absent that no one pushes the brake pedal, as that will result in the cylinder popping out its piston and lots of hydraulic fluid. Not fun.
Okay, that’s enough for today. Tomorrow we’ll get into the weeds as far as putting on a new pair of shoes – which let me tell you is a lot less painful than when you are a kid shopping with your mom for Sunday go to church footwear.
Images ©2013 Hooniverse/ Robert Emslie