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.