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What Is A Master Cylinder?
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What Is A Master Cylinder?

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.
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:
Front/rear (two front and two rear)
Diagonal (left-front/right-rear and right-front/left-rear)
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.
Simply put, the mechanical pressure exerted on the brake pedal by your foot gets converted into hydraulic pressure by the master cylinder. That pressure sends the fluid through your brake lines and engages the pistons at each of the four wheels, thus activating the brake calipers and slowing or stopping your vehicle.
Symptoms of a Bad or Failing Brake Master Cylinder
The brake master cylinder is one of the most important components found in modern car braking systems. It serves as the main valve that pushes brake fluid through the brake lines so the brake calipers can squeeze the pads against the rotors. It functions by pushing a metal rod through a cylinder to force fluid through the braking system to the wheels. One end of this rod is attached to the pedal and is actuated when the pedal is depressed. Usually, a faulty brake master cylinder will produce one of these 4 symptoms that alert the driver of required servicing.
1. Abnormal brake pedal behavior
One of the first symptoms commonly associated with a bad or failing brake master cylinder is abnormal brake pedal behavior. The master cylinder is the component that generates all of the pressure for the braking system, and if it develops any sort of problems sealing or distributing pressure, this may be felt in the pedal. With constant use over time, the seals inside of the cylinder can wear out and form internal leaks. A bad brake master cylinder may result in a pedal that feels mushy, spongy, or slowly sinks to the floor when depressed.
2. Contaminated brake fluid
Another symptom of a bad brake master cylinder is contaminated brake fluid. Brake master cylinders use rubber seals that can break down and wear out over time. When they do, they can contaminate the brake fluid and will turn it a dark brown or black color. Aside from contaminating the fluid, a brake master cylinder with worn seals will not be able to hold brake pressure as effectively and may result in a mushy pedal or one that slowly sinks to the floor.
3. Leaking brake fluid
Brake fluid leaks from the master cylinder or unsecured reservoirs on the cylinder holding the fluid lower critical brake fluid levels. The brake master cylinder needs adequate levels of fluid to exert the right amount of hydraulic pressure to slow down the car. You’ll need to have the brake master cylinder replaced in this situation. If left unresolved, your ability to slow down the car will be impaired.
4. Check Engine Light comes on
Another symptom commonly seen for newer vehicles is an illuminated Check Engine Light. The braking systems on newer vehicles may have brake fluid level and pressure sensors installed in the master cylinder. These sensors are meant to detect any problem with the vehicle’s brake fluid pressure, which is generated by the master cylinder. If they detect that the pressure has dropped, it is possibly due to a problem with the master cylinder. Such issues may also ignite a Brake Warning Light too.
What Does A Brake Master Cylinder Actually Do?
We should probably start by saying a brake master cylinder technically isn’t essential. You could use cable-actuated brakes, if you liked, like the cheap items you get on entry-level mountain bikes. On the other hand, to stand the strain of stopping one or two tonnes of metal, plastic and humans over 10-20 years, the cable would have to be massive.
There are more practical solutions, chief among which are hydraulics. The fact that liquid doesn’t compress makes it perfect for transferring force from one part of a system to another. When it comes to the brakes in your car, the master cylinder is the key component in making that happen.
How to Buy a Good Quality Clutch Master Cylinder
The master cylinder in a manual transmission system functions similarly to the brake master cylinder, and in fact, both systems use brake fluid to lubricate the interior components and provide the pressure necessary to operate the other elements of the mechanism.
The clutch master cylinder contains a “clutch fluid” reservoir, which actually just holds brake fluid. When the clutch is depressed, pistons put pressure on the fluid, which then travels to the slave cylinder, and the pressure in turn allows you to engage the clutch and change gears. When this cylinder goes bad, whether due to wear and tear, clogging, or a leak somewhere in a seal, the transmission will malfunction and this puts your safety at risk.
A few different things can happen when the cylinder is bad. You may experience a spongy-feeling clutch, or the pedal may go to the floor. The clutch may also suddenly engage as you are driving, sending the car lurching forward – this could easily cause an accident. The cylinder should be replaced as soon as the damage is discovered.
When shopping for a new cylinder you want a good balance of durability and price. Because this is such a vital part of your safety while driving, it’s worth a little extra investment.
How to make sure you’re getting a good quality clutch master cylinder
Look for cast steel, rather than extruded aluminum. Cast parts are generally stronger and more reliable.
Choose a trusted name brand for your clutch master cylinder. This is not the time to take chances on an unknown source.
Look for a warranty. Some brands, as well as some sellers, offer warranties – even lifetime – on clutch master cylinders. Calculate the value of price versus the length of warranty to get the right balance for your budget.
If you're still unsure of what to buy, as a good suppliers, we supplies top-quality clutch master cylinders to our certified mobile technicians. We can also install a clutch master cylinder that you've purchased.
Clutch Master Cylinders
The clutch master cylinder is directly connected to the clutch pedal and serves as the portion of the hydraulic system that generates hydraulic pressure. Master cylinders are divided into two primary categories based on the internal mechanism used to generate hydraulic pressure: seal over port systems and center-feed type systems.
Seal over port system
The seal over port system uses a push rod attached to the clutch pedal to produce piston movement in the master cylinder. When the piston moves, seals attached to the piston move forward. The primary seal passes over a small port in the wall of the cylinder, which allows fluid flow to and from the reservoir. Once the primary seal passes over the port, the fixed portion of fluid contained between the master and slave cylinder is moved toward the slave cylinder. This results in disengagement of the clutch. One variation of the seal over port is a plunger type master cylinder. Rather than using the attached seals that move with the pistons, the piston instead moves through the seals.
Center-feed type system
The center-feed type system uses small hooks attached to the nose of the piston. When the clutch pedal is not depressed, these hooks engage a flange on the valve stem. A circular rubber seal on the opposite end of the valve stem is held away from the master cylinder body. This allows fluid flow to and from the reservoir. When the clutch pedal is depressed, the hooks no longer hold the valve stem and it is pushed to the bottom of the master cylinder body. The reservoir is now separated from the fixed fluid, contained between the master and slave cylinder. As the clutch pedal is further depressed and the master cylinder piston moves even further, fluid is forced toward the slave cylinder, resulting in the disengagement of the clutch. Master cylinders are also subdivided into two additional categories: Integral reservoir systems and remote reservoir systems. The fluid reservoir of an integral reservoir system is part of the master cylinder body. The fluid reservoir of a remote reservoir system is attached to the master cylinder body via a rubber hose. The use of integral or remote reservoirs is dictated by the available room in the engine compartment. Both integral and remote reservoir types are used in seal over port and center feed type systems.
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