Have you ever pressed on your brake pedal, expecting the normal, confidence-inducing deceleration, only to be met with a spongy pedal, warning light in your dash, or lack of stopping power? These are all key signs that your brake master cylinder may be failing you.
Ignoring these, and other symptoms of a faulty brake master cylinder places you and others at serious risk. So continue reading to help confirm the issue and get an idea of how much it’s going to cost to fix.
Importance of the Brake Master Cylinder
The brake master cylinder is the heart of the hydraulic brake system in vehicles. This important component converts the mechanical force from the brake pedal into hydraulic pressure in order to activate the brake calipers and pads.
The master cylinder contains a reservoir that stores the brake fluid. When you push your foot on the brake pedal, it activates the push rod inside the master cylinder. The push rod then forces two pistons outward, which pressurizes the brake fluid in the reservoir.
This pressurized fluid travels through the brake lines and hoses to each wheel cylinder and caliper assembly.
Inside the wheel cylinders at each wheel, the pressurized brake fluid applies equal force to both brake pads, clamping them against the rotor surface. This is what allows your vehicle to slow down and stop.
The harder you press the brake pedal, the more force is applied to the pistons in the master cylinder, and the higher the pressure sent to brake calipers to stop the wheels from spinning.
Bad Brake Master Cylinder Symptoms
Once the brake master cylinder starts to fail (or has already stopped working), you’ll likely notice one or more of the symptoms below.
1) Brake Warning Light
The first symptom that is the easiest to notice is when the Brake Warning Light illuminates on the dashboard. This indicates that there is some kind of problem with the braking system, but it might not necessarily mean that the brake master cylinder is at fault.
But if the braking system sensors detect the brake fluid pressure is dropping, it will likely be due to a bad brake master cylinder. This will result in the warning light coming on.
If the ABS light comes on, the root cause could be the brake master cylinder, especially if the master cylinder is leaking fluid.
2) Brake Fluid Leak
The brake master cylinder needs a certain level of brake fluid to create the hydraulic pressure necessary for slowing down the vehicle.
If the brake master cylinder is leaking braking fluid or if there are unsecured reservoirs on the cylinder which are holding the fluid, then you will almost be guaranteed of having a low brake fluid level.
This will impair your ability to slow down the vehicle. You would need to replace the brake master cylinder in this situation.
3) Spongy Brake Pedal
When the brake pedal starts to feel spongy as you place pressure on it with your foot, this will automatically be a sign that your brake master cylinder may be having issues.
The cylinder contains rubber seals which keep the brake fluid inside of it. If these rubber seals were to get worn out or damaged, then there’d be an internal brake fluid leak. The result of this would be a spongy feeling in the brake pedal.
4) Contaminated Brake Fluid
Another problem that could happen as a result of worn out rubber seals is contamination in the brake fluid. The seals not only help keep the brake fluid from coming out, they also prevent dirt and debris from mixing with the brake fluid.
If this were to happen, the brake pressure would not be as strong as you step on the brake pedal. You’d probably end up pressing down harder on the pedal just to get the vehicle to slow down like normal.
5) Sinking Brake Pedal
Following all these other symptoms, you will start noticing the brake pedal not returning to the top after you’ve removed your foot from it. Instead, it will slowly sink to the floor.
This could become a real driving hazard, so you’ll want to fix the brake master cylinder right away at this point.
6) Bad Brake Bias
Brake master cylinders often contain two separate circuits that separate brake fluid to the wheels in pairs. This is to prevent a total loss of braking if there were to be a leak in one side of the system.
Often these circuits will control one front wheel and the opposite rear wheel. For example, the front left and rear right wheels may share a circuit, while the front right and rear left wheels share a separate circuit.
If one circuit has failed, you may notice the vehicle pull to one side under braking. This will be most apparent when you brake really hard, as the front brakes do more work to stop the vehicle than the rear brakes.
7) Uneven Brake Pad Wear
Caused by a failed circuit, if only two out of four wheels are able to apply the brakes, you will notice uneven pad wear. For instance, the front right side and the rear left side would be worn down more than the front left and rear right.
A failure of one of the piston seals in the master cylinder or a brake line leak could cause one circuit to fail. If you notice uneven braking, uneven pad wear, or your car pulls to one side when braking, your brake master cylinder could be at fault.
Brake Master Cylinder Replacement Cost
If you have a brake master cylinder that goes bad, the average cost to replace the cylinder will be between $340 and $520. The cost of the part itself will only be around $100 to $210. But the biggest expense of the replacement job will be in the labor costs, which are around $250 to $320.
If you were to be knowledgeable in vehicles and braking systems, then you would be able to save a fortune on the labor costs by doing it yourself. However, it is a job that requires you to have a good amount of automotive repair experience or else you may make things worse.
Many master cylinders can be replaced by unbolting the master cylinder from the firewall, disconnecting the brake lines, and reinstalling the new unit. The brake system needs to be bled after this procedure. Refer to a repair manual for the proper bleeding procedure.
Do note that ABS systems can be tricky to bleed. If you’re not comfortable performing this repair yourself, please bring your vehicle to a trusted mechanic.
Diagnosing a Faulty Brake Master Cylinder
If you suspect your brake master cylinder may be having issues, there are a few things you can look for and tests that can confirm whether or not it needs to be replaced.
Start by visually inspecting the master cylinder reservoir under the hood to see if the brake fluid level is low or if there are any obvious leaks around the device. Low fluid levels or leakage indicate a potential issue with the seals inside the cylinder.
Next, apply the brake pedal with some pressure and hold it down, taking note of how the pedal feels. If the pedal sinks all the way to the floor and has a spongy/soft feel, the master cylinder may be failing to build up pressure properly. Healthy master cylinders keep the pedal firm and high off the floor.
If you feel comfortable in your ability, you can also test brake system components one by one to pinpoint if the master cylinder is faulty. Use a hydraulic pressure bleeder to maintain equal pressure at each wheel.
If the calipers and wheel cylinders are working properly with this assisted pressure, it points to inner seals in the master cylinder leaking and preventing full system pressure.
While diagnosis can take some time and effort, catching issues early prevents safety hazards and more extensive brake repairs down the road. Reaching out to a professional mechanic can help accurately troubleshoot and identify if replacing the master cylinder is required.