Bad Brake Master Cylinder

The master cylinder generates all of the pressure used in the braking system. Since the cylinder has such an important job in both manual and automatic transmissions, it must be working for your braking system to work. Without brakes, your car is not drivable. If you think you are having problems with the brake master cylinder read on to learn how to diagnose and repair.

What Does the Brake Master Cylinder Do?

The brake master cylinder plays a central role in the automotive braking system. Whenever you press the brake pedal in your car, a plunger inside the brake master cylinder pumps brake fluid through break lines to the brakes. The brake fluid serves as pressure to the brake pads. The brake pads squeeze the brake discs or push the brake shoes to the brake drums. Either of these instances will cause your car to slow down. The performance will quickly degrade.

Important: If you want to save $100s in servicing, diagnosis, and repair costs, improve your car's performance significantly and increase its value by 1.2x with little effort, download our Beginners Auto Maintenence & Repair Manual now. 

Symptoms of Bad Brake Master Cylinder

When the brake pedal is depressed, the force transfers to the brakes using brake fluid. A significant positive relationship has been found between the brakes, the brake lights, and rear end crashes, especially car crashes which happen at night. If you are not sure if your brake master cylinder is failing or gone bad, look for these signs:

Check Engine Light Activated – When the brake master cylinder goes bad, the check engine light (CEL) may activate. This happens especially in newer model cars. Sensors inside the master cylinder detect issues and report them to the computer in your car. Drops in brake pressure are commonly reported. Other circuits involved may also trigger the Check Engine Light (CEL). This may also mean that there is more wrong with the braking system than the brake master cylinder.

Brake warning light – Some cars come equipped with a brake warning light on the dash. Most times, the brake warning light activates when the brake fluid is low or leaking, the brake pads are worn, or the brakes themselves need to be replaced. However, it may also activate when the brake master cylinder is causing problems.

Contaminated brake fluid – When the rubber seals on the brake master cylinder wear out, the brake fluid can become contaminated. When the fluid goes bad, it turns to a darker color. The brake pressure is also affected by the worn seals.

Abnormal behavior from brake pedal – When the brake master cylinder malfunctions, it shows in the brake pedal. The brake pedal will feel mushy when you press it when the brake master cylinder is gone bad. The pedal may also sink all the way down to the floor when the brake master cylinder is bad. Any abnormal feeling when you push the brake pedal is a sign you should check the brake master cylinder.

Causes of Bad Brake Master Cylinder

The brake master cylinder is the heart of the automatic braking system along with brake fluid. When every component of the brake master cylinder is not working at its best, problems arise which must be addressed. here are a few things that can negatively affect the brake master cylinder performance:

Wear and tear –The brake master cylinder is needed for your braking system to work. Although these types of parts are generally built to last for the lifetime of the car. Yet, sometimes any automotive part can malfunction and some must be replaced. As a vehicle is continuously driven and accumulating miles some of the parts eventually wear out. Actually, all auto parts are subject to wear and tear over time. The more you drive the car the more wear and tear you can expect.

Leaks – The vale and piston seals inside the brake master cylinder can start to leak after the car has been driven for many years. The brake fluid absorbs water through air over time. The flat sealing washer may become hardened and fails to seal. In these cases, the leaks can be identified and the seals can be replaced.

Lack of maintenance – Your car should be serviced every once in a while so that the brake fluid can be flushed from time to time. The bore inside the master cylinder becomes corroded which causes leaks in the seals. You can see that the color of the fluid in the reservoir is no longer a healthy color.

If you want to master cylinder problems or other issues and save $100s of dollars that you’ll spend at the auto repair shop, you’ll need to service your vehicle often – you can use our mechanic-rated Auto Maintenance and Repair Manual to do this. It’s basically what mechanics use to go through your vehicle to check if there are any problems that need fixing. As soon as they notice the most minor problem, they’ll ask you to fork out some money even though it’s a problem you can fix yourself in minutes – the manual will teach you how to maintain your vehicle every few thousand miles and it’ll teach you how to fix minor problems that mechanics will ask you to pay for; saving you money in the long run.

A lot of our readers have the Auto Maintenance and Repair Manual printed on their garage wall and 92% of them haven’t visited the auto repair shop in the last year because they know what to do to avoid problems. All it takes is giving your vehicle a little attention every few thousand miles and you’ll never spend money at the workshop again.

What to Do About Bad Brake Master Cylinder

The first step to fixing the problem is to identify what is not working. Make sure it is the brake master cylinder before you start any repairs. Here are a few things you can do to start fixing the problem the right way:

Check the OBD2 codes – When components in your braking system go bad, the computer in your car makes a record of each event. You can use an OBD2 Scanner (view on Amazon) to check the trouble codes that have been thrown about your vehicle. The troubles codes help to diagnose many potential problems under your hood. More than one code may be thrown for your braking system as well as for other components that are associated with your brakes.

Fix it yourself -If you were able to check the OBD2 codes and you’re ready to get it fixed, do it yourself. Two of the most common options for fixing a bad brake master cylinder is to replace the seals and to bleed the brake system. Here are the steps to replace the seals in the brake master cylinder and to bleed the brake fluid from the system:

Replace the seals

The seals in the brake master cylinder can be replaced with a few tools. If you want to try to replace the seals yourself, gather a set of tools and a towel and follow these steps:

  1. Make sure your car is parked in a place where it will be undisturbed.
  2. Open the hood. Use your owner’s manual, if necessary, to locate the master cylinder. It should be on the rear driver side of the engine, mounted on the brake booster cylinder.
  3. Place a towel under the brake master cylinder to catch the fluid. Disconnect the brake lines from the brake master cylinder with an adjustable wrench and loosen the nuts with a tube wrench.
  4. Now remove the mounting bolts which secure the brake master cylinder to the brake booster cylinder with the wrench or a ratchet and socket.
  5. Lift the brake master cylinder and place it on a clean table or workbench.
  6. Plug the brake line openings with pieces of cloth to prevent any contamination.
  7. Empty the brake fluid into a plastic container. Then remove the fluid reservoir from the cylinder with a screwdriver.
  8. Use snap ring pliers to remove the lock ring which secures the pistons in the brake master cylinder at the back of the assembly.
  9. Now remove the retaining ring which secures the top valve.
  10. Tap the master cylinder on the surface of the table in order to slide the seals, pistons, and valve assembly out of the body of the brake master cylinder. Keep all of the parts organized once you get them out so you know how to reassemble them.
  11. Make sure you have the same seals as the ones you are replacing. Then, replace the bad seals with the new ones.
  12. Put everything back in the body of the master cylinder exactly the way it was before you took it out.
  13. Bleed the master cylinder.
  14. Put the brake master cylinder back in the car. Connect the brake lines with the nuts being very careful not to damage the thread. Reinstall the mounting bolts and tighten everything up.
  15. Bleed the brake system
  16. Add brake fluid.
  17. Check the brake master cylinder for any leaks.

Bleed the brake system

Typically, the brake fluid is designed to last for the lifetime of your car, or close to it. However, sometimes it must be changed. The fluid may have lost moisture resistance which means it has begun to absorb water. Air even becomes trapped in the brake fluid. When the fluid is not healthy, it causes big problems in the braking system. Here are the steps to bleed the brake fluid and refill with new fluid that will perform correctly:

  1. Check your owner’s manual to confirm the right types of brake fluid. It is not a good idea to mix types. You can also ask the auto parts store clerk which kind you need for your car’s make and model. Purchase two or three of the 12 ounce cans of brake fluid.
  2. Use a jack to raise your car up on a garage floor or somewhere equally as flat. Remove all four wheels. It is not advised to prop the car up on cylinder blocks. If you do not have the proper jack, take your car in to a repair shop.
  3. Find the four caliper bleeding screws and try to gently remove them. If they don’t loosen, use an oil loosening spray to get them off without stripping them. Only have one screw loose at a time to keep the unnecessary air out.
  4. Check the fluid level in the brake master cylinder reservoir. If you are not sure where it is, check your owner’s manual. The fluid level should be at the “full” mark.
  5. Place tubing over the bleeder screw for the first brake you intend to bleed. Hang the catch container directly above the bleeder screw.
  6. Have someone else pump the brake pedal several times until resistance can be felt after pressing the brake pedal. Maintain the pressure.
  7. Open the bleeder screw. After the fluid is drained close the bleeder screw.
  8. Check the fluid level in the master cylinder reservoir.
  9. Repeat these steps on each bleed screw as many times as necessary to regain pressure in the brakes with clean brake fluid.
  10. Make sure the bleeder screws are replaced tightly and there is no fluid eruption when you use the brakes.

Get a professional inspection– If you don’t have an OBD2 scanner, you can take your car to a local automotive professional who has one. Most car shops have computerized tools to perform diagnostics on your vehicle. You can also make sure that your brake master cylinder is working properly. You can also prevent an escalating problem by having regular maintenance checks on your vehicle.

Safety First

Remember, the brake master cylinder is a very important part of your brake system. If it has gone bad, you should not drive the car until the repair has been made. It is never safe to drive a vehicle with a bad brake light. Many accidents occur when the vehicle is not fully functional. Fix the problem so that you can be back on the road again, safely.