How do you Test a Brake Master Cylinder?

In the braking system, the most important component is the brake master cylinder. It controls the important functions of the brakes, from hydraulics 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 long way toward preventing potential problems with your vehicle. The key function of the brake master cylinder is to pressurize the hydraulic fluid in the car’s braking system. If the right pressure isn’t reached at the right time, things could go wrong with 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, place a rag or container beneath the master cylinder to prevent brake fluid from running onto your wires. Refill your reservoir with fresh brake fluid. Go and apply moderate pressure to your pedal; if the feel 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:

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 are located in 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 noise from the brake and the 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 in testing a master cylinder for internal leaks is to inspect the area around the master cylinder. That is: Check for any fluid leaks; they 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

Have someone press 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 fluid level in the brake master cylinder starts to drop after an initial stop when the pedal is flattened, the master cylinder needs to be replaced or investigated.

On-bench brake master cylinder test.

This type of test is done on a vice; it is usually carried out on new master cylinders, rebuilt ones, or after removing your old master cylinder from your car. Tighten your master cylinder in the vice, and use your impact screwdriver or a giant screwdriver to apply pressure to the plunger. If the plunger is very strong or immovable, it indicates that the master cylinder is still good. But if the plunger keeps going in, it indicates an internal leak or a faulty master cylinder.

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 air in the cylinder and help ensure the brake pedal has safe, efficient brake system functionality.

This method of bleeding can be carried out 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 at the edge with the fitting holes. Brake fluid is corrosive, so ensure you wear 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 your owner’s service manual to determine the correct 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, use a block-off plug to block the hydraulic port that is dripping. 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 with a sharp 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 from there.

Step 6: Another good tip is to gently tap your cylinder with pliers or a screwdriver; you will notice little bubbles rising to the reservoir tank. Once the bubbles stop and you are unable to push the Piston, 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 system. It works in conjunction with all other brake components.

We have cleared the air on how to test a brake master cylinder. If you have any questions or concerns about the brake master cylinder, feel free to let us know in the comments below.