Unlike the front brakes on your car, the rear brakes only perform about 20-30 percent of the braking action. Where this causes an issue is when the rear brakes are not performing at their peak due to worn shoes or a bad wheel cylinder. Because they don’t handle the majority of the braking, it can be a long time before you notice that they are not working until it is too late. Most newer vehicles have disc brakes in the rear, but there are a lot of cars on the road that still have rear drums. Disc brakes squeal when the pads are low, so they give you a warning, drums do not. Brake shoes are the most common wear item on a drums, but they are not the only piece that fails. The wheel cylinder is the mechanical actuator for the drum brakes, essentially the caliper for drums.
How A Wheel Cylinder Works
When you press the brake pedal, the brake fluid is pushed through the lines and into the wheel cylinder. The fluid pushes out on the two plungers, which push out on the upper half of the brake shoes. The shoes push out on the spinning drum to slow the vehicle. When in proper adjustment, drums brakes work well. When they are in disrepair, they don’t work at all.
The wheel cylinder is a very simple device, and unlike the disc brake caliper, which is fully sealed, the wheel cylinder is not quite fully sealed. They have a bad habit of leaking, which is how they fail. The cylinder is made of five components: the housing, a spring, two pistons, the plungers, and the seals. There are two plungers, each resting inside a seal on the end of the housing. The seal is a rubber derivative that contains the pistons and spring inside the housing. Brake fluid is hygroscopic, meaning it absorbs water from the air. Over time, the moisture level in the air leads to wet brake fluid. You can tell your brake fluid has too much water in it when it becomes a dingy brown color. That brown is from the rust that the fluid is causing in the lines and components. Over time, the rust eats away at the inside of the wheel cylinder, allowing the fluid to leak past the seals, and you no longer have functional rear brakes.
Do You Have A Bad Wheel Cylinder?
The main tell-tale sign of a bad wheel cylinder is a leak on your tires. There are two types of fluid that you can see on the inside of the rear tires: brake fluid and gear oil. Brake fluid is very thin and slippery, while gear oil is thick and more sticky. Both have peculiar smells, but gear oil is downright nauseating.
If you have a gear oil leak, that means your wheel bearings and seals are bad, this is a different animal that must be addressed immediately, as they can lock up while you are driving, and that is very bad. For the wheel cylinder, failure is very common on vehicles that sit for long periods of time, and then get driven again. We recently experienced this with a 1981 Chevrolet C10 truck that had sat undriven for several years. After getting it running and driving, the driver side rear wheel cylinder began to leak. After we replaced it, a couple weeks later the passenger side cylinder let go as well, which is fairly common for them to both start leaking at the same time. There are two ways to rectify a bad wheel cylinder: replacement or rebuild. In most cases, a new (remanufactured) wheel cylinder is just a few dollars more than a rebuild kit, but there are some hard to find wheel cylinders that are an exception. Choosing to buy a complete new wheel cylinder, remanufactured wheel cylinder, or just rebuilding the one you have is all up to personal preference. For the C10, we picked up a complete remanufactured wheel cylinder at our local NAPA store and replaced it. The basic swap is simple, but we have some tips and tricks for you to help make it easier.
Tips For Replacing A Bad Wheel Cylinder
Most of the time, the brake drum will come off easily, but if it does not, you have to close up the spreader at the bottom of the drum. This is done through the back of the drum, there is an oval hole at the bottom. Use a drum brake adjusting tool (or flat screwdriver in a pinch) to work the adjuster wheel. You have to do this by feel.
First, you have to remove the wheel and brake drum. Make sure your vehicle is supported with a jack stand and not just a jack. Removing the wheel cylinder is a two-bolt operation, along with the hardline, but you have to get to it first. As with any project like this, you must safely support the vehicle with a jack stand, not just a jack. The hard line on the back of the cylinder can be rusted on, these are really tough. A line wrench is the preferred tool to remove them, but you should always soak them with some penetrating oil before getting started. Let it sit for at least 5-10 minutes. Spray it before you start on the inside components. If the nut strips, you will need to replace the hardline. Sometimes you can get away with just a new flare and line nut, but if the hard line doesn’t have any bends you can straighten to get the extra length, replacement is required.
Once the drum is off, take some pics of the mechanism. Some of this has to be disassembled to get the cylinder out, and if you have never done it before, you are going to want a picture. Also, it is important you only do one side at a time, that way you can always have a look on the other side if you get lost.
Be careful when removing springs, not only can they come loose and fly across the room, but they are really good at pinching fingers or hitting you in the face.
Once the cylinder is replaced, the installation is the reverse of the removal. Just make sure you get all the springs and levers back in place.
Bleeding the new cylinder only takes a few minutes, but you need a partner. With the master cylinder full, have your helper slowly pump the brakes, hold the pedal, and then you loosen the bleeder screw, let the air out, and then tighten the screw. Let your helper know when to pump, when to hold, and when to fold ‘em, err, to release. Bleed the system until there is no more air. It is a good idea to move as much of the old fluid out of the system, as it is likely full of water and rust. Working with drum brakes is not hard, you just have to pay attention and follow our tips and tricks, and you will back on the road, rolling to the beat of your own drum.
Check out all the brake system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more wheel cylinder replacement tips, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.