The brake master cylinder is a vital component in a vehicle’s braking system, responsible for converting the force of the brake pedal into hydraulic pressure that activates the brake calipers.
Without a properly functioning brake master cylinder, a vehicle’s braking power and responsiveness can be significantly reduced, putting the safety of the driver and passengers at risk.
How Does A Brake Master Cylinder Work?
The brake master cylinder is a device that converts the brake pedal’s force into hydraulic pressure used to activate the brake calipers. The brake master cylinder is typically located near the brake pedal and is connected to the brake pedal through a push rod.
When the brake pedal is pressed, the push rod pushes the piston inside the brake master cylinder, forcing brake fluid through the brake lines and into the brake calipers.
The brake calipers press the brake pads against the rotors, which slows down the rotation of the wheels and stops the vehicle. Inside the brake master cylinder are two separate chambers, one for the front and the other for the rear.
When the brake pedal is depressed, both chambers are pressurized simultaneously, allowing the brake system to function as a whole.
The brake master cylinder also has a reservoir that holds brake fluid, which is necessary for the system to work. When the brake pedal is depressed, the brake fluid is forced out of the reservoir and into the brake lines, pressuring the brake calipers.
As the brake pads press against the rotors, the brake fluid is forced back into the brake master cylinder’s reservoir, completing the cycle.
Where Is the Master Cylinder Located?
Typically, the brake master cylinder is attached directly to the firewall (what separates the engine compartment for the car interior) on the driver’s side and in close proximity to the brake fluid reservoir.
Simply find the brake fluid reservoir and follow the brake lines that lead to the master cylinder and brake booster.
Brake Master Cylinder vs Brake Booster
Both the master brake cylinder and brake booster are important components of a vehicle’s braking system and they work together to help you stop your car but that’s where the similarities end.
While the master brake cylinder is responsible for distributing brake fluid to the brakes, the brake booster makes it easier to apply the brakes by amplifying the force you apply to the brake pedal.
So with a failure of the first, your vehicle would not be able to stop. On the other hand, if the brake booster were to fail, it would simply make it more difficult to apply the brakes which would lead to longer stopping distances.
If either of these components experienced complete failure, a serious accident could be the result. But with the brake master cylinder, the situation would likely be much more serious.
Bad Brake Master Cylinder Symptoms
All of your braking operations hinge on the brake master cylinder working well. Your driving safety depends on it. If your car’s master cylinder is going bad, there are several warning signs to watch out for.
#1. Brake Fluid is Contaminated.
There are rubber seals on brake master cylinders that get worn out after a while. If the rubber seals get in too bad of shape, the brake fluid will become contaminated and turn a darker color.
Another effect of worn rubber seals is the inability for the master cylinder to sustain the amount of brake pressure placed on it. As a result, the brake pedal will feel mushy and start falling to the floor slowly.
#2. Sinking Brake Pedal.
Normally, when you press on your brake pedal, it should ideally feel slightly firm and consistent. However, if your master cylinder is failing, you may notice that the pedal gradually sinks to the floor as you keep applying pressure.
This happens because brake fluid is being pushed out of the system with pressure, indicating an internal leak within the master cylinder.
#3. Spongy Brake Pedal.
A spongy feeling brake pedal is another common symptom of master cylinder failure. This occurs when air enters your brake lines, causing the pedal to feel soft and less responsive than usual.
A small leak in the rubber seals of the master cylinder can introduce air into the brake lines and in turn cause a mushy feeling brake pedal which reduces the effectiveness of your braking system.
#4. Low Levels of Brake Fluid.
If the brake master cylinder has a low level of brake fluid, you will likely have some braking issues. The first thing you’ll notice is the brake pedal feeling soft as you lift your foot up on it.
If there are no fluid leaks and the brake fluid level look normal, then your master cylinder is probably the culprit. The brake fluid in the reservoir needs to be sealed up in there and any hose connection needs to be tight and secure.
If you find that there’s a leak from the brake master cylinder and are sure it’s not a loose hose, cracked reservoir, or leaking lid, get the brake cylinder replaced asap.
#5. Brake Warning Light.
If you notice the brake warning light illuminate in your dash, you may have an issue with your brake master cylinder.
That said, this light can also indicate other unrelated problems within your braking system, so you’ll want to have a professional mechanic inspect your brake system to determine the exact cause.
#6. Car pulls to one side when brakes are applied.
When the car pulls to one side when brakes are applied, it could also be the master cylinder. In a situation like this, the car’s brakes aren’t getting pressure evenly on both sides, and the brakes grab and pull the car one way or the other.
#7. Check Engine Light Illuminates.
Many newer cars have master cylinders with pressure sensors and brake fluid sensors built into them. These sensors will detect if there is something wrong with the brake fluid pressure of the vehicle.
If it finds the pressure dropping, for example, then the master cylinder is likely the cause and a diagnostic trouble code will be recorded by the car’s ECU and this will set off the “Check Engine” warning light on the dashboard.
If this light were to illuminate then it could possibly be because of the master cylinder. Of course, you won’t know the exact cause of the check engine light until you get your car scanned.
But if the light appears alongside any other symptom here, there’s a chance it’s your master cylinder. Using an OBD2 scanner is the only way to confirm what error code is stored.
Please Note: If you examine each exterior component and don’t see any signs of problems with them, then your brake master cylinder is definitely going bad. It may even have failed already. If you test your braking system and look for the symptoms listed above, you will be able to tell if your brake master cylinder is truly to blame or if there is some other component to blame.
What Can Cause the Brake Master Cylinder to Fail?
Wear and Tear
Brake master cylinders, like other parts of your car, wear out over time. The internal seals and components simply break down and wear out over time.
This is a natural process, and with regular use, even well-maintained brake master cylinders will eventually need to be replaced.
Contaminated Brake Fluid
Brake fluid is as important to a vehicle’s braking system as what motor oil is to an engine. While brake fluid doesn’t get contaminated as quickly as engine oil, it still requires periodic changes.
If you allow brake fluid to deteriorate for too long, contaminants like water, dirt, or other debris can cause damage to the master cylinder as they can break down the rubber seals and cause corrosion.
Manufacturing Defects
While rare, manufacturing defects can occur in brake master cylinders, especially if it has already been replaced with an aftermarket (non OEM) part.
If a faulty component was used during manufacturing, the brake master cylinder may fail earlier than expected.
How Much Brake Fluid Does a Master Cylinder Reservoir Hold?
In most vehicles, the master cylinder reservoir holds between 8 to 12 ounces of brake fluid. However, this amount may vary depending on the make and model of your car.
To find out the exact capacity of your master cylinder reservoir, you’ll need to consult your vehicle’s owner’s manual or service manual.
When checking the brake fluid level, pay close attention to the fluid’s condition and the reservoir’s markings.
Your reservoir should have a “MAX” and “MIN” line, indicating the recommended limits. Both underfilling and overfilling can lead to brake system complications.
Inspecting the Brake Master Cylinder
When you suspect that your vehicle might have a bad brake master cylinder, you’ll want to get it inspected right away. Luckily, you can do this on your own by following these steps.
- First, make sure to park your car on a level surface and engage the parking brake.
- Pop the hood and locate the brake master cylinder. As noted above, it’s usually mounted on the firewall near the driver’s side, and it’s connected to the brake booster.
- Examine the components for any visible damage, such as cracks, leaks, or rust.
- Check the brake fluid level in the reservoir, as low brake fluid could indicate a problem with the master cylinder.
- Test your brake pedal behavior. A bad brake master cylinder might cause the pedal to feel mushy, spongy, or slowly sink to the floor when depressed.
- Pay attention to any illuminated warning lights on your dashboard. If you notice a brake warning light, it can indicate master cylinder failure.
If you’re not confident about inspecting the master cylinder yourself or simply don’t have time, seek help from a professional mechanic (or that neighbor who’s always working on their car).
Brake Master Cylinder Replacement Cost
The average cost of replacing a brake master cylinder can vary greatly, depending on several factors, such as the make and model of the vehicle, the type of brake master cylinder used, and the labor cost.
On average, the cost of a brake master cylinder replacement can range from $300 to $800, with most of the cost being labor. It’s essential to get an accurate estimate from a qualified mechanic, as the cost can vary greatly depending on the specific circumstances of the replacement
How long does it take to replace a brake master cylinder?
Replacing a brake master cylinder is a laborious process where not only will your mechanic need to swap out the brake master cylinder, they’ll also have to do a bleed (to remove air bubbles from the system) and to adjust the pushrod if necessary.
Expect that the full process will take anywhere from around 1.3 hours to about 3.4 hours.
Mechanics’ per-hour rates across the United States now range from $110 to $175.This means that the labor cost for a brake master cylinder replacement could be as low as $143 (1.3 hours at $110/hour) and as high as $595 (3.4 hours at $175/hour).