brake master cylinder

Understanding brake master cylinder

One of the most essential parts of a car’s braking system is the brake master cylinder. It’s the first part of a car’s braking system that is activated by pressing the brake pedal. It is in charge of converting the driver’s pressure on the brake pedal into hydraulic pressure that is then sent to the brake callipers, which in turn causes the wheels to slow down or stop. The brake master cylinder functions as a kind of pump that compresses brake fluid before sending it to the callipers through the brake lines.

brake master cylinder

When the brake master cylinder fails, it might be challenging or even impossible to slow down or stop the car, which raises the chance of accidents. The brake master cylinder needs to be frequently inspected and maintained to ensure optimal operation and prevent any potential road dangers. For the brake master cylinder to survive the pressure and heat produced while braking, high-quality materials like cast iron or aluminium are typically used.

Well, in this article, I’ll be discussing the meaning, functions, location, diagrams, components, types, working, and problems of a brake master cylinder. I’ll also discuss the symptoms of a failing master cylinder and its replacement cost.

So, let’s dive in!


What is a brake master cylinder?

A brake master cylinder, also referred to as a master cylinder, is a hydraulic pump. It feeds brake fluid into the brake circuit in order to convert the pressure on the brake pedal to hydraulic pressure. In other words, a brake master cylinder is a component of a vehicle’s braking system that converts the mechanical force a driver’s foot exerts on the brake pedal into hydraulic pressure. The brake callipers or wheel cylinders then transmit the hydraulic pressure produced by the brake master cylinder to the brake pads or shoes, which are used to slow down or stop the vehicle.

The brake master cylinder is a vital part of the braking system and is essential for providing safe and efficient stopping power. Even though the master cylinder is intended to last the lifetime of the vehicle, like brake callipers, it occasionally develops leaks or experiences other failures, necessitating rebuilding or replacement.

Read more: Understanding automotive braking system


The function of a master cylinder also referred to as the master brake cylinder is to feed brake fluid into the brake circuit and regulates this in accordance with the mechanical force to convert the pressure on the brake pedal to hydraulic pressure.

Well, a master cylinder’s typical functions include developing pressure that causes the wheel cylinder pistons to move toward the rotors or drum after the shoes or pads have already generated enough friction. It also keeps the system full of fluid as the brake linings decrease and can maintain a slight pressure to prevent contaminants from entering the system.

Location of  the master cylinder

The master cylinder for manual brakes is directly attached to the firewall and connected to the brake pedal. In power-assisted brakes, the master cylinder is attached to a brake booster, which gives the braking system extra power. The brake pedal is connected to the booster by the assembly, which is fastened to the firewall in the engine compartment.

Diagram of a brake master cylinder

Diagram of a brake master cylinder

brake master cylinder diagram

 Components of a brake master cylinder

Here are the main parts of a master cylinder:

Brake pedal

The master cylinder assembly’s crucial component, from which you activate the master cylinder, is the brake pedal.


The piston inside the cylinder rotates when you press down on the brake pedal. As a result, the piston causes the brake fluid inside the cylinder to compress, producing high hydraulic pressure. Pushrods and return springs hold the MC piston in place.


In hydraulic braking systems, brake fluid is typically stored in the reservoir. Usually, it is made of cast iron, aluminium, or occasionally plastic. Rectangular reservoirs are usually used for rear disc brakes; however, they can also occasionally be round. It is possible to see the oil level and colour in this reservoir because of its design.


It is the air-tight housing in which the piston moves when the brake pedal is depressed, changing and multiplying the force. Either cast iron or aluminium is used to make the cylinder.

Piston assembly

A piston assembly is essentially a set of parts that consists of a piston, a number of o rings or gaskets, also referred to as cups or o rings, circle P, a primary cup, a secondary cup, and a return spring. The master cylinder’s bore is not far from where the piston assembly is situated.


Usually, a master cylinder with an open system contains this part. Because it is comprised of thin rubber, it can expand and compress and deform. On one side, it carries liquid and, on the other, air.

Brake fluid

Brake fluid is typically hydraulic fluid. It is seen as being equally essential to engine oil. Brake fluid’s main job is to make it possible for a car to apply pressure to the brakes, slow down, and stop.

Series of gasket

A circle P, a primary cup, a secondary cup, and an O ring make up the series of gaskets. The piston is held in place between the pushrod and the cup by a circle P that serves as a retainer.

When the brake lever or pedal is depressed, the primary seal allows brake fluid from the reservoir to flow into the brake hose pipe, but the secondary seal keeps the brake fluid sealed and prevents it from flowing to the outside when the brake pedal is depressed.

Spring return

It is a specific kind of spring that is installed inside the cylinder and aids in keeping the piston and brake pedal in their original positions after the brake pedal has been released. Drum brake and disc brake assemblies both use it.


This valve connects the brake line to the vehicle. The compressed brake fluid is then further transmitted to the calliper with the use of this valve.

Types of brake master cylinders

Here are the main types of brake master cylinders:

Open system

The reservoir tanks of an open-system master cylinder assembly contain a bladder. These bladders aid in adjusting the master cylinders’ brake fluid level. Because the bladder only has one exposed side, it responds to temperature changes by expanding or contracting. An open system has the benefit of automatically adjusting the braking fluid level in the master cylinder assembly. protection against the disc brake system heating up.

It has no impact on braking performance because the bladder’s expansion or contraction automatically adjusts the amount of brake fluid in the system. Automatic adjustment is a drawback of an open system. When the brake pads fail, the open system adds an excessive amount of fluid to the calliper bore, causing the piston to adjust to the failed pad.

The disc and calliper assembly could sustain damage in this situation. Additionally, it makes noise and impairs brake performance. The performance of the brakes can be impacted by both an underfilled bladder under vacuum and an overfilled bladder under pressure. It is therefore regarded as old technology.

Closed system

There is no bladder in a reservoir tank in a closed-system master cylinder. Therefore, human adjustment is required for the system. Such a system has the benefit of allowing the brake fluid level to be checked through the reservoir tank and replaced if it is low or does not meet specifications. In such systems, worn brake pads can be quickly identified and replaced.

The drawback of such a system is that it might lead to overheating issues. The effectiveness of braking may be impacted if the brake fluid level is not optimal. Braking performance may be impacted if an air bubble develops anywhere in the system. To prevent the production of air bubbles when changing the brake fluid, the brake fluid has to be bled.

Single cylinder

The piston inside the cylinder is pushed by the brake pedal lever in this type of master cylinder, forcing fluid into the slave cylinder through the lines. A spring that is inserted within the cylinder forces the piston back to its initial position when the brake pedal is removed.

The brake fluid reservoir’s brake fluid is drawn out of the lines and into the cylinders by the generated negative pressure. Because only one piston or circuit is used, single-cylinder engines exert the same amount of force on all of their wheels.

Tandem or dual-cylinder

A dual-cylinder-dual-piston or single-cylinder-dual-piston cylinder is used in this modified version of the master cylinder for independent braking between the front and rear wheels. It allows for independent braking of the vehicle’s front and rear wheels, which is crucial for safety.

When the brake pedal is depressed, the primary piston presses against a spring that is attached to the rear of the secondary piston. The secondary piston starts to pump fluid through its designated system as the spring is compressed. These are nearly always utilized in automobiles since they are more effective than single-circuit master cylinders.

Portless master cylinder

In comparison to conventional designs that make use of a compensating port, the portless kind offers a quicker brake release. When the brake is released, a valve assembly in the piston opens to equalize pressure.

This makes it possible to do without a compensating port, which restricts fluid flow and lowers braking system pressure during the initial application. These perform best when used with the ABS system, which modifies the brake force quickly.

Read more: How much brake fluid do I need? (What you need to know)

Working principle of a brake master cylinder

The working of a brake master cylinder is a basic principle of hydraulic systems in automobiles, which can be relatively easy to understand, but the application and components involved can be quite extensive and cover a broad range of topics.

The brake master cylinder is made up of numerous moving parts, and, being the heart of the brake system, its durability and good condition are essential. Each component’s function will be clarified, making it simple to pinpoint a problem.

When the driver presses down on the brake pedal, a pushrod transfers the force to the brake master cylinder. The brake master cylinder’s piston is actuated by the pushrod, compressing the brake fluid inside the cylinder and producing hydraulic pressure.

The brake lines then transfer the hydraulic pressure from the brake master cylinder to the brake callipers or wheel cylinders at the wheels. By pushing the brake pads against the rotors or drums and creating friction, the hydraulic pressure inside the brake callipers or wheel cylinders at the wheels, which contain pistons, stops the car.

To ensure that the brakes on one axle keep working even if one hydraulic circuit fails, the brake master cylinder includes two separate hydraulic circuits, one for each axle. Additionally, the brake master cylinder has a reservoir that stores brake fluid and makes up for any volume loss brought on by system pressure changes.

The system’s brake fluid is made to be incompressible, or not change volume when pressure is applied. Regardless of how hard the brake pedal is pressed, this property of brake fluid ensures that the same pressure is maintained throughout the entire braking system.

Watch the video below to learn about a master cylinder

Read more: Understanding the working of the anti-lock braking system (ABS)

Problems with the master cylinder

Here are some issues with the brake master cylinder that you should avoid;

  • Leaks: A master cylinder leak can result in the loss of brake fluid and a drop in brake pressure, which can impair braking ability or result in failure.
  • Worn Piston Seals: The master cylinder’s piston seals may become brittle and worn over time, which can result in leaks and poor braking performance.
  • Air in the brake lines: It might be difficult to apply adequate braking force when the brake pedal is soft or spongy due to air getting into the brake lines.
  • Contaminated Brake Fluid: The master cylinder may become damaged as a result of dirt, moisture, and other contaminants getting into the brake fluid, which could result in leaks and decreased performance.
  • Master Cylinder Failure: Rarely, the master cylinder itself may fail, rendering the vehicle completely unstoppable.

To ensure safe braking performance, it’s crucial to get your master cylinder inspected and fixed as soon as you suspect a problem.

Read more: Understanding Brake Calliper

Symptoms of a failing master cylinder?

Bad Signs and symptoms

Here are signs and symptoms you’ll notice when your brake master cylinder is failing:

Strange Brake Pedal Behavior

You’ll probably first notice that the brake pedal is functioning strangely. The pressure required to brake is entirely produced by the master cylinder. Therefore, you will notice it in the pedal if there is a problem with pressure distribution or sealing.

Leaks might also develop if the cylinder’s seals continue to deteriorate. That contributes to the brake master cylinder’s role in producing a mushy or spongy pedal. You might even be able to get it to touch the ground.

Unfortunately, you can’t tell if the master cylinder is damaged just from this symptom. The most frequent cause of a spongy pedal is air in the brake lines. Your car might only need to have the brake fluid changed.

Read more: What causes a spongy or soft brake pedal (and how to fix it)

Brake fluid leak

Check for any brake fluid leaks in the system. You have a problem if you see brake fluid dripping along the brake booster or firewall or down the back of the master cylinder. Even brake fluid leaking down the cabin’s firewall is possible. The system will eventually run out of brake fluid due to this leak, which will lessen your stopping power.

Read more: How to change brake fluid

Inconsistent Brakes

The brakes may start to behave strangely as the master cylinder begins to fail. Your brakes may first function normally and then suddenly stop working entirely. This is undoubtedly a very serious matter because it becomes more dangerous when you are unsure whether the brakes are functioning properly. The pedal may sometimes feel solid one moment and sink to the floor the next.

Decreased Braking Ability

The brakes may stop working entirely in the front or back when the master cylinder starts to fail. You will observe a considerable loss in braking power when this occurs. You may end up in an accident if you don’t allow for this extra time as you prepare to stop. This is due to the fact that a failing master cylinder can increase the stopping distance.

The same symptom can be brought on by other broken brake system components. You can be dealing with old, worn-out fluid or air in the brake lines. These signs also show up if your brake line or hose has burst.

No Brakes

The brakes might not engage at all in the worst scenarios. Although it doesn’t frequently happen, it is possible. When you push the brake pedal, the master cylinder acts as a hydraulic pump to continuously pressurize the brake lines. This design makes it considerably more likely for either the front or the rear brakes to fail, but not both. However, if the master cylinder fails, you can end up with no brakes at all.

Brake master cylinder replacement and its cost

The following shows how the brake master is replaced;

  • Disconnecting the brake lines and brake booster.
  • Take the old master cylinder off of the firewall by unbolting it.
  • Reattaching the brake lines and booster, and installing a new master cylinder.
  • Flushing the brake system of old fluid and air.


The average cost for replacing a brake master cylinder is between $240 and $520. While parts are priced between $160 and $320, labour costs are estimated to be between $98 and $173. Taxes and other costs are not included in this range, nor are your particular vehicle or geographic area taken into account. There might be additional repairs required. Depending on the car model and make the master cylinder replacement may cost:

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

Read more: Brake pad and rotor replacement cost


What are the symptoms of a faulty brake master cylinder?

The pedal will be impacted if it fails since the cylinder won’t distribute pressure effectively. A bad cylinder will wear down and begin to leak if you keep driving your car with it. When pressed, the pedal will also feel mushy and spongy and may sink considerably.

What does a brake master cylinder do?

The master cylinder, also known as the master brake cylinder, feeds brake fluid into the brake circuit and regulates this in accordance with the mechanical force to convert the pressure on the brake pedal to hydraulic pressure.

What causes a master cylinder to fail?

The master cylinder, a crucial component, converts the movement on your brake pedal into hydraulic pressure. This master cylinder eventually fails as a result of extensive pressure-related wear and tear that occurs over time.

When should a brake master cylinder be replaced?

The brake pedal is spongy or soft. Your brake master cylinder’s components will degrade over time.
The brake or check engine light is illuminated.
Your brake fluid appears dirty or black.

Can the master cylinder be repaired?

A soft or spongy brake pedal likely indicates your master cylinder has to be “rebuilt.” You will need to take it apart, clean the metal components, refine the bore, replace worn-out parts with new ones from a rebuilding kit, and then put it back together.

How do you test a brake master cylinder?

To press and hold the plunger in the master cylinder’s back, use a screwdriver. Above a few millimetres, the plunger should be extremely firm, if not completely immovable. If the plunger continues to move in, at least one of the internal seals is broken.

Can a master cylinder fail without leaking?

Yes, master cylinders can fail without leaking. The clutch master cylinder has a piston inside, and different manufacturers place the high-pressure line and return (low-pressure) line in the cylinder in different locations.

How long do master cylinders last?

The master cylinder will ultimately lose its functionality, much like any mechanical or hydraulic equipment. The standard master cylinder can last anywhere between 60,000 and 200,000 miles, depending on usage. For example, highway commuters use their brakes less frequently than city taxis, so their master cylinders last longer.

How do you reset a master cylinder?

  • Remove the master cylinder.
  • Take off the rubber boot.
  • Remove the circlip.
  • Take out the piston and spring.
  • Examine and clean the cylinder.
  • Fit the new seals to the new piston.
  • Install the replacement spring.



The brake master cylinder, which transforms the driver’s pedal input into hydraulic pressure that engages the brakes, is a crucial part of a vehicle’s braking system. For safe and dependable braking performance, it must be operating properly. The safety and efficiency of a car’s braking system depends on regular maintenance, quick inspections, and taking care of any master cylinder issues.

That is all for this article, where we discussed the meaning, functions, location, diagrams, components, types, working, and problems of a brake master cylinder. We also discuss the symptoms of a failing master cylinder and its replacement cost. Hope it was helpful. If so, kindly share it with others. Thanks for reading, see you around!