The wheel cylinder plays a major role in your car’s drum brake system.
Its job is to apply the brake shoes onto your brake drums, which in turn, slow down your car.
But what exactly is the wheel cylinder?
What are the things that can go wrong with it, and most importantly, what can you do about it?
In this article, we’ll dive into all the ins and outs of the wheel cylinder, cover some common problems with it (with an easy solution), and answer some wheel cylinder FAQs.
This Article Contains
· What Is A Wheel Cylinder?
· The Anatomy Of A Drum Brake Wheel Cylinder
· How Does The Wheel Cylinder Work?
· What Are The Symptoms Of A Faulty Wheel Cylinder?
· Why Does The Wheel Cylinder Fail?
· Wheel Cylinder FAQs
What Is A Wheel Cylinder?
The brake wheel cylinder is a key component of your car’s drum brake system.
It’s positioned at the top of the wheel inside the drum brake and is mounted to the drum brake backing plate with a bolt.
What is it used for?
It’s used to push a pair of brake shoes outwards so that they can contact a brake drum to slow down your vehicle with friction.
How is this different from a disc brake?
Unlike the drum brake wheel cylinder that delivers a pushing force onto a brake shoe, a disc brake caliper squeezes a brake pad onto each side of a spinning rotor to slow the car down.
How common are drum brakes?
While most modern vehicles use disc brakes, it’s still common for an older car or a small truck to still have drum brakes on their rear tires.
Now that we’ve covered the basics, let’s go into some detail over how wheel cylinders are structured. It’ll make it easier for you to understand how things can go wrong later.
The Anatomy Of A Drum Brake Wheel Cylinder
The wheel cylinder structure is relatively simple.
Its main body is a cylinder with a bore, typically made of cast iron or aluminum.
The cylinder bore is fitted with these:
· A piston on each end which connects to a brake shoe via a shaft
· Each piston has an internal piston seal (or piston cup) to maintain hydraulic pressure and prevent brake fluid from leaking past the piston
· A spring between the pistons that holds each piston seal in position
· External dust boots (also called a dust cap) cover each end of the wheel cylinder to protect the cylinder bore from moisture, brake dust, and dirt
In addition to those, there are two other important components:
· An inlet port that connects the wheel cylinder to a brake line that carries brake fluid
· A bleeder screw that’s used to bleed brake fluid and eliminate the brake cylinder of air
Now that we understand the structure of a brake wheel cylinder, let’s get to how the wheel cylinder functions with the rest of the braking system.
How Does The Wheel Cylinder Work?
When you depress the brake pedal, the force generated by your foot transfers to the brake master cylinder.
The master cylinder then converts this force into hydraulic pressure, which is carried by brake fluid through a brake line to each wheel cylinder.
This pressurized brake fluid in the wheel cylinder then pushes the cylinder pistons outwards, pressing each brake shoe against the rotating brake drum, stopping the wheel.
When the brake pedal is released, return springs pull the brake shoes away from the brake drum, pushing the wheel cylinder pistons back into their bore.
FYI: This dual piston design isn’t the only type of wheel cylinder. Some drum brake configurations use two single piston wheel cylinders — one at the top of the drum and one at the bottom, each connected to a brake shoe.
What Are The Symptoms Of A Faulty Wheel Cylinder?
A bad wheel cylinder is often hard to spot because it’s located inside the drum brake.
However, here some signs that something’s wrong with it:
· Your car has poor brake response, and you’ll find that braking takes longer
· Your brake pedal feels mushy, soft or the pedal sinks to the vehicle floor
· There’s a brake fluid leak in your rear brake drum that pools near a rear wheel
· The rear drum brakes drag or lock up
If you run into any of these problems, don’t drive your car.
Why Does The Wheel Cylinder Fail?
Since a wheel cylinder contains so many moving parts, there are plenty of reasons why this hard-working component can fail.
Here are five of the most common ones:
1. Rubber Seal Failure
The wheel cylinder piston seals and dust boots are made of rubber.
These seals become brittle with time and can fail with exposure to extreme heat or due to natural wear and tear.
When they fail, your vehicle will leak brake fluid, the hydraulic pressure will drop in the braking system, and your braking performance will be compromised.
2. Worn Pistons
Pistons are meant to be a specific diameter relative to your cylinder bore size.
However, with time, pistons can wear down and won’t fit snugly in the cylinder bore anymore. If this happens, there’s a risk of a piston seal leaking or the piston rocking, which can increase wear.
3. Stuck Pistons
The cylinder bore is supposed to have a smooth surface.
However, corrosion and pitting can develop in the cylinder bore due to exposure to moisture in the brake fluid, which can cause your piston to get stuck.
Stuck pistons result in a drum brake that won’t release, creating major braking problems in your drum brake system.
4. Pistons That Slip Out Of The Bore
Excessive brake drum wear can create enough space for pistons to slip out of the cylinder bore completely. At this point, your drum brakes won’t work at all.
5. Cracked Cylinder Body
Older wheel cylinders can split and crack under pressure, leaking brake fluid, compromising its function.
Note: While it seems like a relatively simple component, replacing or fixing a bad wheel cylinder isn’t a straightforward task because of its location and how it works with the rest of your drum brake system.
So what’s the best way to get it fixed?
Wheel Cylinder FAQs
Here are a few answers to questions you have about wheel cylinders.
1. How Many Wheel Cylinders Does A Car Have?
That depends on how many drum brakes your vehicle has and the brake cylinder configuration used.
In general, a vehicle with drum brakes will have 2 dual piston wheel cylinders.
That’s because cars with drum brakes typically only use them as rear brakes.
2. How Long Does A Wheel Cylinder Last?
You can expect your wheel cylinders to last around 3-5 years, or up to approximately 100,000km. This will depend on the make and model of your vehicle and the type of driving done.
Heavy-duty driving (like towing or driving in mountainous terrain) will definitely wear down your wheel cylinder faster.
3. Will My Brakes Still Work If The Wheel Cylinder Fails?
Yes, your brakes will still work, but you’ll experience poor brake response.
Most cars have a dual circuit braking system — meaning if one circuit fails (as in a rear wheel cylinder goes bust in a wheel), there’s still braking capability in the other circuit.
However, keep in mind that your brakes won’t be as powerful, the braking distance will be longer, and if you have disc brakes on the front, the rear of your car might jump if you brake hard.
4. If My Wheel Cylinder Leaks, Do I Need To Replace The Brake Shoes Too?
A brake shoe replacement is only necessary if they are too thin or saturated with brake fluid from a leak.
If there’s not too much fluid on the brake shoe, it can still be cleaned up quickly and reliably.
5. Is It More Cost Effective To Replace The Wheel Cylinder With The Brake Shoes?
For the most part, yes.
If you replace the wheel cylinder during a brake shoe job, the new wheel cylinder and labor cost will typically be included in a package price.
The labor time spent replacing the brake shoes and wheel cylinder overlap, so a wheel cylinder replacement is a relatively small additional labor charge.
However, most professional technicians would recommend a replacement as many aftermarket wheel cylinders match OEM specifications these days, so a rebuild is not needed.
Additionally, note that rebuilding requires tons of care and time, specialized mechanical knowledge and is not always doable if your wheel cylinder is too damaged.