Wheel cylinder: A thorough guide

The wheel cylinder is a small cylinder located at each wheel brake. This cylinder is a part of the hydraulic drum brake; it comes in a grey cast iron or aluminum housing.  The cylinder contains the brake fluid's wheel brake to exert hydraulic pressure that forces the brake shoes against the brake drums and stops the vehicle. These may begin to leak on older vehicles and hinder the brakes' performance but usually are inexpensive and relatively easy to replace.

It has a cylinder, two pistons, two rubber cups, and a spring. The fluid presses against the pistons that move outward in the cylinder; when the pistons come closer, the liquid forces its way into the master cylinder, the spring between the two pistons holds the rubber cups in positions.

Wheel Cylinder

Types of the wheel cylinder

There are two types of this device.

  1. Single-acting
  2. Dual acting

Single-acting wheel cylinder

Single-acting cylinders use a single-piston, meaning that the force they generate is one-directional only. It is useful on some non-servo drum brakes. Each wheel cylinder only has one piston, so the bore is closed off at the opposite end. There are two of these cylinders on each wheel assembly, one for each brake shoe.

It is made of the same materials and has an aluminum piston, a lip seal, a spring and expander, a dust boot, a pushrod, and a bleeder screw.

Dual acting wheel cylinders

Dual-acting cylinders use two pistons opposite of each other, meaning that the force acts in two different directions. Most modern vehicles use double-acting wheel cylinders because they are simpler to design, install, and bleed.

Dual-acting wheel cylinders use a common with a piston and lip seal in each end. There is usually a coil spring with expanders on each end, positioned between the lip seals. The expander helps to hold the seal lips against the cylinder bore when there is little or no hydraulic pressure.

How does it work

Wheel cylinder mechanism

The primary functions of the wheel cylinder include:

  • Releasing hydraulic pressure from the master cylinder.
  • Pushes the two pistons inside each wheel cylinder outward.
  • Forces the brake shoes against the drums and apply the brakes.
  • When you release the brake pedal, return springs pull the shoes back away from the drums and push the wheel cylinder pistons back into their bores.

The wheel cylinder is a simple device that is not fully sealed, and it has a bad habit of leakage, which leads to how it fails. It is equally made up of five components, the housing, a spring, two pistons, the plungers, and the seals.

The plungers each rest inside as seals on both ends of the housing, the seal is rubber-like, and it contains the piston and spring inside the housing. The brake fluid absorbs water from the air (hygroscopic). Over time, the moisture level leads to wet brake fluid, and the fluid color becomes dingy brown color, coming from the rust that the fluid causes in the lines and components. The rust eats away the inside of it, allowing room for leakage.

When to replace a wheel cylinder

Wheel cylinder

The brake pedal is depressed to the floor

The first sign of a bad wheel cylinder is a depressed brake pedal. When the brake wheel cylinder loses its ability to supply brake fluid pressure to the brake shoes, the pressure inside the master cylinder is compromised. This is what causes the brake pedal to go all the way to the floor when pressed.

In some instances, this is caused by a brake line that is loose, damaged, or broken; but the most common reason why the brakes go to the floor is that the rear brake wheel cylinder is broken.

You hear lots of noise from the rear brakes

If you hear loud, grinding sounds coming from the rear of the vehicle as you stop, it's an indication of two potential problems. Either the brake shoes are worn out and digging into the brake drum, or the brake wheel cylinder is losing brake fluid pressure and not smoothly applying the brake shoes.

Internal or external leak

It may develop any internal or external leak, their ability to pressurize and extend the piston may be compromised.

Functioning one side only

The brake wheel cylinder may function on one side but not the other. This causes one of the shoes to apply pressure and the other one to stay put. Since the system works harmoniously, the lack of dual pressure can cause sounds similar to grinding or worn-out brake shoes.

Poor brake response

A vehicle with poor brake response will be harder to stop, especially in heavy braking situations.

How to replace  A wheel cylinder      

If you intend to replace your wheel cylinder yourself, then you'd need to follow the steps below:

  • Remove battery cables from the positive and negative terminal. When replacing any mechanical components, it's always recommended to remove power from the battery.
  • Raise the vehicle on a hydraulic lift or with jack stands. If raising the rear axle with jack stands, make sure to place wheel chocks on the front wheels for safety.
  • Remove rear tires and wheel. We recommend replacing the brake wheel cylinder in pairs, especially when replacing other rear brake components.
  • Remove the drum cover. The drum cover usually slides right off the hub without removing any screws.
  • Detach the retainer springs with vice grips.
  • Separate the rear brake line from it.
  • Remove brake wheel cylinder bolts on the back of the wheel hub.
  • Pull the old one from the vehicle.
  • Remove old brake shoes.
  • Clean rear and interior of the rear hub with brake cleaner
  • Have brake drums turned or surfaced and replaced if worn out
  • Install new brake shoe. After the brake housing has been cleaned, you'll be ready to reassemble the brakes.
  • Install the new wheel cylinder. Once you install the new shoes, you'll be ready to install the new rear cylinder.
  • Bleed the brakes. Since you removed the brake lines and the brake wheel cylinder does not have brake fluid inside, you'll have to bleed the brake system.
  • Reattach the wheel and tire.
  • Lower vehicle and torque rear wheels.
  • Reconnect battery.

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