Disk Brake – Master Cylinder Assembly (Part 3)

by mahavir on November 25, 2010

Master Cylinder is an equal important part like brake disk/rotor in disk brake assembly. Generally it is made of aluminum or cast iron. It acts like throttle system acts in any motorcycle or automobile. As throttle supplies fuel to engine, a master cylinder supplies brake fluid to caliper assembly. A master cylinder assembly has various parts like reservoir tank, bore, piston assembly, lever or pedal, hosepipe etc. Let’s have a look to master cylinder assembly.

Lever or Pedal: It is that part of master cylinder assembly from which you give instructions to master cylinder.

Push rod: The push rod is connected to a lever and a piston. When a brake lever is pressed it pushes piston to slide inside the bore.

Bore: As we have seen in caliper assembly a master cylinder assembly also has a bore under which a piston moves

Piston Assembly: A piston assembly has a piston, series of O ring/gaskets (Also known as cup/O ring, circle p, primary cup, and secondary cup) and a return spring. A piston assembly is housed inside the bore of master cylinder.

Piston: A piston or mc piston is connected to a push rod and a return spring. It slides inside the bore when a brake lever or pedal is pressed to push brake fluid.

Return spring: It is the same thing as we have seen in drum brake and disk brake assembly. It helps piston to move back in its original position when a brake lever or pedal released.

Series of O ring/gaskets: It includes cup or O ring, primary cup, secondary cup and circle p.  A cup and Circle p (also known as lock ring) act as a retainer for the piston. They are positioned between push rod and pistons. A primary seal can be a cup style or an O ring style seal. When a brake lever or pedal is pressed this cup allows the brake fluid of reservoir to flow over the top of it, into the brake hose pipe. On the other side the secondary seal keeps the reservoir brake fluid sealed and prevents it to go other side of cylinder when a brake lever or pedal is pressed. It also acts as a spring retainer.

Reservoir: It is made of aluminum or cast iron. It contains brake fluid. It has one window from which you can check the oil level and color of oil. Generally the shape rectangular or square size but some time round shape reservoir used for rear disk brake.

Brake Fluid: Brake fluid is a type of hydraulic fluid. It is as important as oil is important in engine. It transfer force into pressure. As engine oil, brake fluid also graded like DOT 3, DOT 4 etc. DOT stands for Department of Transportation. For more info on brake fluid click here

Bladder: Bladder is found in open system master cylinder. It is made of thin walled rubber which allows it to deform via expansion and contraction. It has liquid one side and atmosphere at the other side.

Hosepipes: A tube type pipe which transfers brake fluid from master cylinder assembly to caliper assembly.

Clamp: It holds the entire master cylinder assembly.

When a brake lever or pedal is pressed, the push rod which is connected to lever or pedal and piston, pushes the piston. This movement allows the piston to slide and push the return spring inside the bore of master cylinder, which generates pressure in reservoir tank. At this moment a primary seal allows the brake fluid of reservoir tank to flow over it into the brake hosepipes. A secondary seal ensures that the brake fluid does not go other side. When a brake lever is released the return spring pushes the piston back into its original position and allows the fluid to flow back to reservoir via hosepipe and cylinder bore.

Now there are two type of master cylinder assembly

1. Open system Master cylinder:

An open system master cylinder assembly has bladder inside the reservoir tank. This bladder helps to adjust the level of brake fluid into the master cylinder. As one side of bladder is exposed to atmosphere it expands or contracts according change in heat.

Advantages of Open System

  • Automatic adjust the level of brake fluid in master cylinder assembly
  • Protect the disk brake system to overheat
  • As the level of brake fluid is automatically adjusted by expansion or contraction of bladder it does not affect the performance of braking.
  • You don’t need to bleed the brake as it is open system.

Disadvantages of Open system

  • The major disadvantage of having open system that it adjusts automatically. When brake pads are wearing out the open system will supply the extra amount of fluid into the caliper bore to adjust the piston according to worn out pads. If this condition remains for long time, it can cause to change disk and caliper assembly which is very costly affair. Apart from this it creates brake noise and affects the braking performance.
  • Having under filled (bladder under vacuum) or overfilled (bladder under pressure) brake fluid can majorly affect the braking performance in open system master cylinder.
  • It is considered as outdated technology.

2. Closed System Master Cylinder:

A closed system master cylinder does not have bladder into reservoir tank. Hence the system requires to be adjusted manually.

Advantages of Closed System Master Cylinder

  • It is widely used in all kind of automobiles
  • You can check the brake fluid level through the reservoir tank and can change if it is dark or not between the predetermined level.
  • If brake pads are worn out it can easily be identified in this kind of system.

Disadvantages of Closed System Master Cylinder

  • It can cause overheating problem
  • If the level of brake fluid is not at predetermined level it can affect the performance of braking
  • If the air remains anywhere in the entire system it can affect the performance of braking
  • You need to bleed (Oil change) if any kind of air remains in the system.

We have covered all the three major parts brake disk, caliper assembly and master cylinder assembly. In the last part we will see the working principal of disk brake, Maintenance tips and advantages and disadvantages of disk brake over drum brake.

If you want to suggest anything else to be covered in the next article regarding brakes let me know.

- Mahavir Kothari

(Image Sources: Image 1, Image 2 and Google images)

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{ 13 comments… read them below or add one }

vinodh November 25, 2010 at 9:51 am

Thanks Mahavir,nicely xplained,and pls enclose Closed system diagram.And another one question.Bajaj pulsar front brake is more effective than Honda’s unicorn.How this happen and which Part of disc brake assembly determines the efficient braking,And How to make effective braking like pulsar in unicorn. Thank u once again


mahavir November 25, 2010 at 10:40 am

Dear Vinod,

The diagram is same. The only difference is rubber bladder which is not there in Closed system.



Mahavir Kothari


Yogesh November 25, 2010 at 10:34 am

Hello Deepak,

Can you please explain how to clean up the disk plate/pad, as it is making unwanted metallic noise, I have recently changed the brake pads of my HH hunk, for 1 wk it was ok but after that it is making metallic noise on applying brakes.
I got back to the showroom and he told that it might be because of some mud particles inside and requires to be cleaned up.
Can you please give suggestion on what the pb is and what is the possible solution?



mahavir November 25, 2010 at 8:45 pm

Dear Yogesh

maintainence tips will be there in part 4 which will answer your query.



Mahavir Kothari


Prashant November 28, 2010 at 1:13 am

Your brake pads are new and probably have not ‘bedded in’ yet. Google ‘brake pad bed in’ and read how to ‘bed in’ new brake pads. After a while, the noise will go away… provided the pads ‘bed in’ properly. Also, mud particles could also be the reason for the metallic noise. Stand the bike on the main stand, hold the front wheel up, spin it as fast as possible and hold a water jet over the caliper/pads. Apply front brake intermittently. Do this for 2-3 minutes and the mud on the pads should be removed.


Yogesh November 29, 2010 at 10:02 am

Thanks Prashant!
Infact I did it what you suggested and now the noise is gone…It was due to mud particles only. I sprayed a high pressure jet on the pad and the problem got solved :)

fas November 25, 2010 at 8:07 pm

Yes I agree with Yogesh, how should we clean it? Because otherwise it makes weird noise.


bikeshunt.com November 25, 2010 at 11:48 pm

An excellent review. I love it. A great knowledge is shared by this bike.


Deepak November 27, 2010 at 12:45 pm

Waiting for the maintenance part of the disc brakes. Till now well explained. Keep us updating.


Prashant November 28, 2010 at 1:07 am

I would, like many other readers here, really appreciate if you could explain (in some detail) the procedure to service disc brake components (caliper and the master cylinder)… a kind of DYI (Do It Yourself) procedure. I own HH Ambition bike and I successfully cleaned the caliper unit some time back. I would like to know how to go about opening the master cylinder components (i.e. piston, return spring, etc). Thank you for your informative articles.


Prashant February 18, 2011 at 12:36 pm

Anyone??? Please…


Yogesh November 29, 2010 at 10:11 am

Question : Why the disc brakes of HH Hunk not so effective as Bajaj bike’s disk brakes?


Ravi September 12, 2011 at 1:56 pm

This article helps me to understand the whole disk brake system of bajaj pulsar and with help of this i myself replaced my faulty brake assembly and saved some money.



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