How do you Test a Brake Master Cylinder?

In the braking system, the most important component is the brake master cylinder. It controls important functions of the brakes right from hydraulics down to all other brake components. How do you test a brake master cylinder in your car? That question is crucial to your vehicle’s functionality.

So, you shouldn’t wait till you get to the auto mechanic to check for a bad brake master cylinder; you can do that easily on your own.

Periodically checking for a bad master cylinder or brake booster goes a very long way to clear potential problems for your vehicle. The key function of the brake master cylinder is to pressurize the hydraulic fluid in the car braking system. If the right pressure is not reached at the right time, then things could go wrong in your brake system.

Any slight disruption in required pressure levels can prove to be very dangerous. Therefore, you might need to carry out the checks yourself.

How to Test a Brake Master Cylinder

It is appropriate for you to know the signs of a bad master cylinder. In this regard, if you’re experiencing a spongy brake pedal, the best way to know if the master cylinder is the cause is to detach the brake lines from the master cylinder and replace it with the same size IOS. To do this, you have to place a rag or container beneath the master cylinder to prevent brake fluid from pouring on your wires. Refill your reservoir with fresh brake fluid. Go and apply moderate pressure on your pedal, if the feel on your pedal is not firm enough, your brake master cylinder is faulty. If revert is the case, your master cylinder is okay. You’ll have checked your brake lines and your wheels for leaks.

Let’s go over how to test the master cylinder:

how to test if brake master cylinder is bad

Locate the Brake Master Cylinder

After opening the hood, look behind the main engine bay. That should give the position of some important brake system parts where you can find a plastic cylinder that is openable, that’s the brake fluid reservoir, which is typically filled with hydraulic fluid. In manual transmission engines, two plastic cylinders lie around the same area. The larger reservoir is for the brake fluid.

Note that using bad brake fluid grades in the brake fluid reservoir will definitely affect the brake master cylinder

Use the Brake Pedal

You can direct someone to sit in the driver’s seat and step on the brake pedal. Then, monitor for noise from the brake and brake fluid level closely. As pressure on the brake pedal mounts, check for a fluid swirl or bubbling in the brake reservoir. If that is confirmed after two or more tries, then the brake master cylinder is not functioning properly and should be replaced.

Check for internal Leaks

The first step on how to test a master cylinder for internal leaks is to carry out a close inspection of the surrounding area of the master cylinder. That is Check if any fluid leaks are noticeable; it might be along the body of the reservoir or near the opening of the cylinder. If that is the case, then you might need to replace the bad master cylinder. If you notice the fluid leak along the brake hose leading from the Brake cylinder, then the brake cylinder probably isn’t the problem.

Flatten the Brakes

Direct someone to flatten the brake pedal for a while as you monitor the engine. Ensure that the brake pedal goes to the floor totally as you do that. If you notice that the level of the fluid in the brake master cylinder starts to drop after coming to an initial stop when the pedal is flattened, then the master cylinder needs to be replaced or looked into.

On-bench brake master cylinder test.

This type of test is done on a vice; it is usually carried on new master cylinders, rebuilt ones, or after removing your old master cylinder from your car. Tight your master cylinder on vice and use your impact screwdriver or giant screwdriver to apply pressure on the plunger. If the plunger is very strong or not movable, it shows that the master cylinder is still good. But if the plunger keeps going in, it tells there is an internal leak, or the master cylinder is faulty.

How to bench bleed your brake master cylinder

If you have ever wondered how to bench test a master cylinder or want to know how to bench bleed a brake master cylinder, we’ll clear the air right now.

It is appropriate to bleed the brake master cylinder before installation, to remove any stored-in air in the cylinder and to help ensure the brake pedal with safe and efficient brake system functionalities.

This method of bleeding can be carried on new and rebuilt brake master cylinders. Of course, we do not encourage getting a rebuilt master cylinder kit.

Step 1: mount your master cylinder on a vice. Ensure you grip it on the edge that has fitting holes. Brake fluids are corrosive, so ensure you put on your PPE, such as gloves and goggles.

Step 2: Get your new brake fluid and fill the reservoir tank to the max gauge. There are various brake fluid types, so ensure you refer to the owner’s service manual to know the right fluid type for your car. The fluid type is most likely written on the reservoir tank cover.

Step 3: You’ll notice the flow of brake fluid from the cylinder outlet ports. The fluid can start dripping out immediately or take several minutes. This solely depends on the master cylinder you purchased. Do not allow the fluid to drop below the minimum gauge, as this will trap air in the system. Once the fluid starts flowing adequately, take a block-off plug and block the hydraulic port that is dripping out. Most new master cylinders are sold with these ports. Wait a few more minutes for the second fluid outlet to start dripping adequately.

You can apply pressure on the master cylinder center bore with an impact screwdriver do not use a screwdriver that has a sharp-end tip.

Step 4: Once the second outlet port starts dripping adequately, depress the plunger in the cylinder and plug in your block-off plugs. Make sure the plugs are well-tightened.

Step 5: The next step is to depress the plunger on the master cylinder. You have to do this several times. Continue doing this until it gets strong to push. This implies that you have pushed the air from the master cylinder back to the reservoir tank – which goes off from there.

Step 6: Another good tip is gently tapping your cylinder with any pliers types or with screwdrivers you will notice little bubbles coming up to the reservoir tank. Once the bubbles stop and you are unable to push the Piston, it means that your bench bleeding is complete.

Do not remove the block-off plugs until you’ve completely installed the master cylinder back in your car. Remove the block-off plugs and fix them in your hydraulic lines. You still have to bleed your wheels to ensure there is no trapped-in air in your brake systems. You have to refer to your vehicle owner’s service manual to know your brake bleeding sequence. For visual clarification on how to do this, kindly check out this video:

Final Words

As you have already seen, the master cylinder is a major component in your brake systems. It works connectively to all other brake components.

We have cleared the air on the question, how do you test a brake master cylinder? If you have any questions or concerns regarding the brake master cylinder, feel free to let us know in the comment box below.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

2 thoughts on “How do you Test a Brake Master Cylinder?

  1. Hey Wellala,

    This can be caused by fluid leaks, air in the system, or a bad master cylinder.

    I’d recommend you bleed the system first, and if that continues, have your mechanic take a closer look.

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