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Understanding Your Car's Master Cylinder

Engine Cylinders, Pistons and Valves
The brake systems found in modern cars utilize a variety of different components in order to safely stop your car. While each of these components has a vital role, none plays a larger part than the master cylinder. The master cylinder converts the movement of your brake pedal into the flow of hydraulic fluid necessary for your brake calipers to stop your car.

Unfortunately, many people fail to recognize the importance of the master cylinder. Even those who do often struggle to understand just how this important component works. If you would like to increase your overall level of automotive brake systems, keep reading. This article will provide a useful introduction to the master cylinder, as well as some of the problems it commonly faces.

Master Cylinder Functions

As noted above, the master cylinder regulates the flow of hydraulic fluid within your brake system. Each time you depress your brake pedal, a push rod in the master cylinder moves. This push rod contains two pistons. As the rod moves in response to your brake, the pistons exert pressure on the brake fluid inside of the master cylinder.

In other words, the pistons push some of the fluid out of the master cylinder. This displacement increases the pressure exerted on the fluid in the rest of your system. Because brake fluid cannot be compressed, the pressure travels all the way through a series of hoses to your brake. There the hydraulic pressure causes the calipers to close around the rotor, stopping your car.

The opposite chain of events happens when you take your foot off the brake. As the pistons retract, fluid flows back into the master cylinder. This relieves the pressure off all of the points downstream. The calipers retract and the brake pads release the rotor, allowing your car to move once again.

Master Cylinder Problems

Leaks present the largest and most common threat to a master cylinder. Leaks fall into two categories: internal and external.

Internal Leaks

Internal leaks tend to affect the pistons and their seals. If a piston's seal wears out, the piston will no longer be able to prevent hydraulic fluid from moving backward when the brakes are applied. In other words, the piston won't be able to generate any pressure in the hydraulic fluid.

Instead of becoming pressurized, the fluid will simply push back around the piston and into the brake fluid reservoir. As you can imagine, this is a highly dangerous scenario since it means that your brake pads won't close around the rotors.

Fortunately, master cylinders come with a built-in failsafe to protect against such internal leaks. Namely, the master cylinder contains two separate circuits. Each of these circuits falls under the control of one of the pistons. Each circuit controls a different pair of brakes in your car. Provided neither piston suffers a leak, when you depress the brake, hydraulic pressure will increase in both circuits.

If, however, either piston becomes leaky, the other piston will continue to generate pressure within its circuit. Of course, in that case, the brakes will only work on half of your wheels. While this should still be sufficient to stop your car, the faulty piston must be repaired as soon as possible.

External Leaks

Master cylinders can also suffer from external leaks — in other words, leaks that allow brake fluid to escape from the system. If the level of brake fluid drops too low, air may get into the system. Air presents serious problems in the brake lines. Unlike brake fluid, air can be easily compressed.

As a result, when you apply your brakes, the pressure of the fluid won't change much. Instead, the air will simply compress into a tighter and tighter space. This problem often manifests as a brake pedal that sinks all the way to the floor of your car without having any appreciable effect.

For more information about what it takes to keep your master cylinder running strong, please contact Greenville's brake experts at Anderson Road Alignment.


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