If your vehicle has soft, squishy brakes that make it hard or slow to stop your car, or if the brake pedal continues to move to the floor with the vehicle stopped, there might be a problem with a part in the braking system, like the brake master cylinder. This article and video reviews common bad master cylinder symptoms, how to tell if the brake master cylinder is the cause of squishy brakes, how to replace the brake master cylinder correctly, and what other parts to check in the braking system if you notice these symptoms.
What Is the Brake Master Cylinder and How Do You Know It’s Causing Soft or Spongy Brakes?
Do I Need to Fix the Brake Master Cylinder Right Away?
The brake master cylinder contains the brake fluid. When you press on the brake pedal, a piston inside the master cylinder forces the brake fluid down the brake lines and to the brakes. The fluid allows the brakes to function, and your vehicle needs brake fluid to stop safely. If there is a problem with the master cylinder and it’s leaking fluid, you’ll need to fix it right away since this reduces the vehicle’s ability to brake.
Will I No Longer Be Able to Brake If the Master Cylinder Is Leaking?
The brake master cylinder reservoir has a larger area for fluid to travel to the front brakes and a smaller area for fluid to travel to the rear brakes. This is for safety. If, for example, you lose function of the rear brakes, you can still stop with the front brakes, although it will be less safe. This way, if fluid is leaking out of the brake master cylinder, there will still be fluid to disperse to the brakes on the car.
Symptoms of a Bad Master Brake Cylinder
Bad brake master cylinder symptoms include:
- Brake fluid leaking from the master cylinder
- Rusted, rotted or moist brake booster
- Brake fluid dripping onto the ground
- Moist parts around the master cylinder from leaking fluid
- Low brake fluid
- Dirty, contaminated brake fluid
- Brakes feel squishy at times or like the brakes don’t want to stop the vehicle
How to Diagnose the Brake Master Cylinder
Check the Master Cylinder for Signs of Leaks
Common symptoms of a bad brake master cylinder include leaking. If the brake master cylinder is leaking, it’s not safe to drive and fix later. Problems with the brake master cylinder require immediate attention and you’ll need to replace the part if it is leaking. A problem like this will require you to tow or move the vehicle to a place where a mechanic can work on it.
The master cylinder can leak around the seals that connect from the cylinder to the reservoir. They can also leak at the plunger that connects to the brake booster. If brake fluid is leaking into brake booster, there will be major problems braking and it will be an expensive fix.
Check the Brake Booster for Dampness, Rust and Rotting
You might not be able to visually see the leak. The seal between the master cylinder and the brake booster could be leaking fluid onto the outside or inside of the booster. Brake fluid is corrosive. It can swell up and corrode paint on the brake booster and rust and rot it out if it’s leaking onto it. It can also contaminate the ground if it drips down.
You’ll need to remove the master cylinder and inspect the inside of the brake booster to see if there is damage. If there is fluid inside, you’ll need to service or replace the brake booster.
Check the Brake Fluid Level and Condition
Low brake fluid and dirty brake fluid can also cause problems with the brakes. The brake fluid should be clear, but if it is dark and dirty and/or has debris, it can cause issues with the braking system.
Contaminated fluid can clog and cause issues when dispersing the brake fluid through the brake system. As the fluid leaks, it can swell up the seal and will continue to leak as the faulty seal worsens. Compression seals on the plunger inside the master cylinder can also get damaged, swell, roll over and leak fluid.
How to Replace the Brake Master Cylinder
If your brake master cylinder is damaged and showing symptoms that it’s defective, it’s time to replace it. Whether a brake master cylinder is showing signs its defective or if you notice a soft, or spongy brake pedal that still moves to the floor with the vehicle stopped, follow these steps to find problems in the brake system.
Steps on how to replace the brake master cylinder correctly
- Inspect and Be Aware of the Condition and Position of the Brake Lines
You want to inspect the brake lines connected to the master cylinder. If the lines are rusted and rotted, and you loosen the fittings from the cylinder, you could twist or damage the lines. This can cause restrictions or another leak after installing a new master cylinder.
Check all brake lines that lead down to the flex hoses, and check the lines that lead to the brakes. Rotted brake lines will cause an issue. Swollen flex hoses in areas with brackets could cause a restriction on the flex hose and restrict the flow of brake fluid to the brakes.
Read more about brake lines
- Inspect the Condition and Function of the Bleeder Screws
Inspect the bleeder screws on the brake calipers or drum brakes, depending on what kind of brakes your vehicle has. Make sure the bleeder screws can open up. If they can’t, you’ll have a situation where you can’t remove the bleeder screw, which will not allow you to get all of the air out of the system after servicing the master cylinder.
Rotted and rusted bleeder screws can be replaced without replacing the brake caliper, but they will need to be replaced whether you replace the entire caliper or not. You will need to be able to loosen and remove the bleeder screws because if you’re replacing the master cylinder, you will need to bleed all of the air out of the brakes. Check all four wheels for good bleeder screws. You may have to replace an entire caliper if the bleeder screw is broken.
Read more on how to remove a bleeder screw - Check the Condition of the Brake Pads
Check the condition of the brake pads. Make sure there is enough brake pad material and that the brakes are in a condition so that they’re reusable.
Read more on how to inspect the brake pads - Inspect the Brake Calipers
You can test the caliper by testing the piston with a rewind tool and see if it functions.
Read more on how to inspect the brake calipers
- Replace or Rebuild a Damaged Brake Master Cylinder After All Parts of the Braking System Have Been Inspected
After you’ve inspected all parts of the brake system, replace the defective master cylinder. This will require you to loosen the brake line fittings and remove the hold-down bolts from the master cylinder.
You could rebuild and fix specific parts of the cylinder yourself with a rebuild kit. You could also replace the defective master cylinder with a new one. - Check the Brake Master Cylinder for Leaking
Inspect the brake master cylinder. Look at the backside and see if there is fluid present or if the part is leaking. If it’s leaking, that can damage the brake booster.
- Check the Brake Booster for Damage
Inspect the brake booster. See if the paint is starting to deteriorate and flake off the booster. The paint is more of an aesthetic problem, but look inside the booster and see if brake fluid managed to leak inside the drum.
If the booster is sealed and it doesn’t seem like fluid made its way inside, remove and brush off any flaking paint from the brake booster.
- After the Repair Is Complete, Bleed the Brake System and Test Drive the Vehicle
Once you’ve replaced the master cylinder, you’ll need to bleed system to remove all of the air trapped inside the system. Once new fluid has been added to the system, buy your specific manufacturer’s bleeding sequence and take the vehicle for a road test
Read more on how to bleed the brakes
Finished Learning About Bad Brake Master Cylinder Symptoms and How to Replace One? Watch More How-to Videos to Fix Your Car
Learn how to do more than identify bad brake master cylinder symptoms and how to replace one. We have thousands of how-to videos specific to hundreds of makes and models with instructions from professional mechanics to help you do many DIY repairs.
.
Read More Automotive Knowledge
- Parking, Corner, Side Marker, Turn Signal, Bumper, Repeater Lights & Reflectors Guide
- What Starters Are, the Different Types, and How They Work
- What Is Automotive Weatherstripping and What Are the Different Types?
- Can You Drive a Car with Leaking Struts?
- What Does a Leveling Kit Do? Leveling Kits Explained
- How to Increase the Range on Keyless Entry
Shop Parts and Tools
- Brakes & Wheel Bearing
- Steering & Suspension
- Headlights & Lighting
- Exterior, Body Parts & Mirrors
- Interior
