(Updated on June 15, 2020)
When you step on the brake pedal in a vehicle to slow it down, its braking system uses a brake master cylinder to convert the pressure you place on the pedal into hydraulic pressure.
The conversion of the pressure takes place because the brake master cylinder transfers brake fluid to the braking circuit as you step on the brake pedal.
The brake master cylinder will do this no matter if your vehicle is using disc brakes or drum brakes. There is no way any braking system could exist without a brake master cylinder.
Bad Brake Master Cylinder Symptoms
Once the brake master cylinder starts to go bad, there will be certain noticeable symptoms that will arise. If you suspect an issue with your brake master cylinder, you should address the problem immediately. It is very dangerous to drive without brakes that work properly. Below are the top symptoms of a bad brake master cylinder that you will surely notice.
1) 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 $320 and $500. 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 $230 to $300.
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.