The master cylinder plays an important role in the proper function of your braking system since it is responsible for helping circulate brake fluid to the brakes. If the master cylinder fails, it could spell disaster for both you and your vehicle.
Some signs that the master cylinder needs to be repaired or that it has failed includes increased stopping distance, unusual brake feel, brake fluid leaks, and the brake warning light illuminating.
What is the Master Cylinder?
The master cylinder is a part of the braking system that contains two pistons connected to two springs and a pushrod. When the pedal is pushed, the pushrod builds up pressure within the system by working the master cylinder pistons, sending brake fluid to the brakes to ultimately operate them and help them exert pressure on the discs or drums.
Many vehicles feature a mechanical braking system, in which there is a physical connection between the brake pedal and the master cylinder via the connected pushrod. Brake-by-wire systems are starting to become more popular in modern vehicles in which the master cylinder is electronically operated without any physical connection, similar to electronic throttle control systems.
Common Signs & Symptoms of a Bad Master Cylinder
Unlike many other systems in a vehicle, a bad master cylinder shows relatively few warning signs of a potential issue. This can be both good and bad; It can help isolate a problem and display specific areas of concern, but if you are not paying close attention, it can potentially limit obvious failure points.
Increased Stopping Distance
Usually, the first sign of a bad master cylinder includes increased stopping distances. This could also be a sign of a different braking issue and should provide a solid heads up to get the brake system inspected. Because you know your vehicle best, this should be fairly obvious to you, but because many things may cause extended braking distance, it is almost like the check engine light of your braking system.
Unusual Brake Feel
Another obvious-to-you sign of master cylinder failure is the brake feel, specifically, if the brake pedal has a loose, spongy, or less powerful feel than normal. Like stopping distance, this is another fairly broad sign of master cylinder failure and should merit an immediate visit to your local mechanic. Again, an unusual brake feel could be a sign of a different braking system problem.
Even though it does not pertain to the braking feel, a sunken brake pedal could also be a sign of master cylinder failure. This could contribute to an unusual pedal feel since familiarity with your own vehicle contributes to the ability to pick out irregularities in feel and appearance, quicker than most mechanics who do not drive your car daily.
Leaking or Low Brake Fluid Levels
If the feel of the brake pedal does alert you to a potential issue, you can perform a simple affirming test by checking the brake fluid. Low braking fluid levels and brake fluid leaks usually go hand-in-hand, so a more pointed sign of master cylinder issues usually includes both low fluid levels and visible leaking. If it is a slow leak, low fluid levels will be the more obvious sign.
Low fluid levels are often caused by the master cylinder’s seal being worn out or by not following changing intervals. If you have not changed your fluid for extended periods, the master cylinder may actually be fully functional. Most fluids vital to your vehicle’s function that are not properly refilled or changed tend to be slowly used up over time.
Uneven Braking or Brake Pad Wear
Many newer vehicles are equipped with a dual circuit braking system that controls separate sets of wheels as a failsafe. This means that two wheels’ brakes fail, there is a second circuit that allows at least two wheels to still have braking power. This is not a very common issue, but it can contribute to uneven brake wear, extended braking times, and even different overall braking feel for intuitive drivers.
Learn More: Signs and Symptoms of Bad Brake Pads
Debris and Other Contaminants in Brake Fluid
A pretty direct sign of master cylinder failure is contaminants with the brake fluid. This often occurs when the master cylinder seal has eroded over time, allowing both parts of the broken down seal and other outside contaminants to enter the system. The result is usually discolored fluid and often, contributes to an unusual brake feel.
Brake Warning Light Illuminates
Many vehicles are equipped with an ABS or brake warning light. Master cylinder pressure sensors can trip this warning, even though it could be illuminated by other braking issues. The light looks like a circle with parentheses and an exclamation point in the center. Vehicles without this warning light can trigger the check engine light.
Total Braking Loss
Total braking loss could occur as a worst-case scenario type of event. Almost one hundred percent of the time, a total braking loss does not usually occur without any previous signs and symptoms. This is a good thing since total brake loss can be catastrophic, but it is also why the signs and symptoms noted above should never be ignored.
What is the Expected Lifetime of a Master Cylinder?
Under normal conditions, master cylinders will last the life of the vehicle if everything is maintained. This is because the overall master cylinder function is quite simple and the master cylinder itself does not contain many moving parts that can go wrong.
Normally, the only things that contribute to premature master cylinder failure are poor maintenance or some kind of physical damage to the seal, brake pedal sensor, or brake connection to the braking system. Keeping up with fluid changes is especially beneficial to the longevity of the braking system, and physical damage is rare unless the seal is manufactured poorly or the damage evaluation because of an accident is not fully executed.
How Much Does the Master Cylinder Cost to Replace?
If the master cylinder fails because of low braking fluid, you can often change the fluid yourself for between $50 and $100. Having a mechanic do this incurs labor costs, often pushing the change closer to the high end of that range and potentially more, depending on the mechanic, make, and model of the vehicle.
If the issue is with the master cylinder itself, it can cost $500 or more to fix, depending on the vehicle and mechanic. This can be done by yourself, but it is recommended to be done by a professional mechanic unless you are especially adept at vehicle care. If done incorrectly, it can make the car worse than when it was initially brought to the shop. The least expensive master cylinder replacements can cost around $300.
Your vehicle’s master cylinder is important because it controls how your vehicle brakes. A faulty or bad master cylinder can cause all kinds of trouble, especially when the signs and symptoms of failure point toward a necessary mechanic visit. Increased braking distance, unusual braking feel, brake fluid leaking, contaminants within the brake fluid, and even total brake failure in a worst-case scenario situation are all signs of a bad master cylinder that should not be ignored.