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Blog Car Care Advice What Is A Master Cylinder? Everything You Need To Know (2021)

What Is A Master Cylinder? Everything You Need To Know (2021)

February 19, 2021

Want to learn about brake master cylinders and how to fix them?

A master cylinder is a central component of your braking system. The entire braking process relies on your master cylinder, and without it, your brakes wouldn’t be able to function.

In this article, we’ll demystify the brake master cylinder for you. 

We’ll explain how it works, what it does for your braking system, how to spot problems with it, and the easiest way to get your brake issues fixed.

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Let’s get started.

What Is The Master Cylinder?

The brake master cylinder is a component in your car’s braking system that drives the pressure generated by the brake pedal to the braking mechanism at your vehicles’ wheels. It’s essentially the heart of your car’s brake system.

When you press down on the brake pedal, that force pushes a piston through the brake cylinder, converting the force into hydraulic pressure.

This pressure pumps hydraulic fluid through the brake lines, transmitting pressure to a secondary cylinder at each wheel’s braking mechanism.

The secondary cylinders drive the caliper piston to engage the brake calipers in disc brakes (the wheel cylinder in drum brakes). This action then goes on to stop the wheel.

The clutch system in manual cars also employs master cylinders, but they’re not the same kind as brake master cylinders.

Where Is The Master Cylinder Found?

For manual brakes, the master cylinder is attached directly to the firewall and linked to the brake pedal.

In power-assisted brakes, the master cylinder is attached to a brake booster, which supplies more power to the braking system. The assembly is attached to the firewall in the engine compartment, with the brake pedal linked to the booster. 

Now that you know how the master cylinder functions in the brake system, let’s see what happens inside the master cylinder:

How Do Master Cylinders Work?

Most master cylinders have a “tandem” design (sometimes called a dual master cylinder). 

In the tandem master cylinder, two master cylinders are combined inside a single housing, sharing a common cylinder bore. This allows the cylinder assembly to control two separate hydraulic circuits.

Each of these circuits controls the brakes for a pair of wheels. 

The circuit configuration can be:

This way, if one brake circuit fails, the other circuit (that controls the other pair) can stop the vehicle.

There’s also a proportioning valve in most vehicles, connecting the master cylinder to the rest of the brake system. It controls the pressure distribution between the front and rear brake for balanced, reliable braking performance.

The master cylinder reservoir is located on top of the master cylinder. It must be adequately filled with brake fluid to prevent air from entering the brake system.

Image Source: Wikipedia Commons

Here’s what happens in the master cylinder when you press down on the brake pedal:

When you release the brake pedal, the springs return each piston to its initial point. 

This relieves the pressure in the system and disengages the brakes.

How The Master Cylinder Influences Braking Performance

The master cylinder bore diameter is an essential factor influencing braking performance. 

A change in the cylinder diameter will affect how the brake pedal feels to you and the brake pressure that’s applied. 

If the cylinder bore diameter is too large, it will create a:

Because of this, you could end up with a jumpy car, as you’ll have to tap the brakes hard (due to the high resistance), and the brakes will engage too fast. 

If the cylinder bore size is too small, it will have a:

You’ll feel less resistance on the pedal, but you’ll be pressing down farther to get the brakes to work. This is not a great scenario in a traffic jam or if you need to make a quick stop.

Note: Drum brakes require less pressure, so the master cylinder for drum brakes is usually smaller. They typically have a one-way residual pressure valve. The residual valve helps maintain a bit of pressure in each brake line, so the drum brakes can engage faster. 

We know what you’re thinking:

Learning about how master cylinders work is great, but how can you tell when it fails?

How To Spot A Bad Or Failing Master Cylinder

A bad brake master cylinder can result in several issues. Here are some common red flags that indicate a faulty master cylinder:

1. Unusual Brake Pedal Behavior

Your brake pedal should reflect any major problems in the sealing or force distribution of your master cylinder. 

For example, you might notice a spongy brake pedal — where it’ll lack resistance and may sink slowly to the floor when pressed. The brake pedal may also not spring back smoothly into place after you remove your foot. This is usually due to an issue with your brake fluid pressure – which is likely caused by a bad brake master cylinder.

As a general rule, take your car to the mechanic whenever your brake pedal suddenly starts acting differently.

2. Brake Fluid Leaks 

Brake fluid leaking under your car is a clear sign that something is wrong. If this happens, make it a point to have your mechanic check your brake fluid reservoir. A leak will cause the brake fluid level to drop.

Luckily, the master cylinder has several seals inside it to keep the brake fluid and brake pressure contained. However, if any piston seal wears out, it’ll create internal leaks. 

A severe dip in your brake fluid level will compromise the performance of your brake system and your road safety.

3. Contaminated Brake Fluid

Brake fluid is supposed to have a clear, golden yellow to brown color.  

If you notice your brake fluid turning a dark brown or black, something’s wrong.

If your brakes aren’t performing up to par, there’s a chance that a rubber seal in the master cylinder has worn out and broken down. This introduces a contaminant into the brake fluid and darkens its color.

4. The Engine Light Or Brake Warning Light Comes On

Newer vehicles may have brake fluid level and pressure sensors installed in the master cylinder. These will detect unusual drops in hydraulic pressure and alert you.

That’s why, if your engine light or brake warning light turns on, don’t ignore it. It could be a sign of master cylinder failure, especially when accompanied by any of the previous symptoms.

5. Weaving When Braking

The brake master cylinder usually has two separate hydraulic circuits to transfer the brake fluid to two different pairs of wheels. Any failure in one circuit can cause the car to drift to one side when braking. 

6. Uneven Wear In Brake Pads

If one of the circuits in the master cylinder has a problem, it can translate to uneven brake pad wear. One set of brake pads will wear down more than the other —  which can again result in your car weaving whenever you brake. 

Read more: Discover the differences between Ceramic and Semi-Metallic Brake Pads

How Much Does A Master Cylinder Replacement Cost?

The typical cost for a brake master cylinder replacement can be anywhere between $230 to $750. Your vehicle’s make and model will drive most of this cost variation, as it dictates the costs of the parts needed.

Here’s what it could cost for some popular makes:

Vehicle Costs Including Parts & Labor
Chevrolet Silverado$300 – $545
Ford Focus$400 – $750
Toyota Camry$310 – $530
Honda Civic$230 – $475

A master cylinder replacement is usually one of the costlier brake repairs because a lot goes into the process.

A master cylinder replacement typically involves:

The Easiest Way To Take Care Of All Your Brake Repair Needs 

Whenever you’re looking for a mechanic to help with brake-related repairs, make sure that they:

Fortunately, there’s an easy way to ensure all of this.

RepairSmith is the most convenient car repair and maintenance solution.

Here’s why RepairSmith should be your number one repair option:

All you need to do is fill an online form to get an accurate estimate for your brake repairs.

A Final Word On Master Cylinders

While master cylinders can seem like a very daunting part of your brake system, the fact is they’re fairly straightforward.

All your inner braking mechanisms depend on your master cylinder, and if there’s a problem with it, your entire braking system might be at risk.

If your master cylinder has an issue, or if you need any kind of brake service, you can always get in touch with RepairSmith to get your car back in good working order!