How to check master cylinder and brake booster

Under normal operation, most car owners never consider the inner workings of the brake system. However, when you press that brake pedal and you notice the car is not slowing, that tends to grab your attention rather quickly. We all understand that the brake system is essential for the safe operation of any vehicle, but what is not commonly known is that a leading culprit of brake system failure in older cars, trucks, and SUV’s is the power brake booster.

The power brake booster is used to supply brake fluid throughout the brake lines, which permits the system to work effectively. If the brake booster fails, it can lead to a soft brake pedal or even complete brake system failure. In the next few paragraphs, we’ll explain how this critical component works within the brake system, and a few pointers to help you diagnose and determine if the power brake booster is the root source of your problem.

How Does the Power Brake Booster Work?

In order to understand how the power brake booster fits into the modern braking system, it’s critical to explain how brakes work. To safely stop your vehicle, three scientific principles need to occur — leverage, hydraulic pressure, and friction. Each of these actions must work together in tandem to stop the vehicle. The brake booster helps to supply the correct hydraulic pressure in order for the brake calipers to apply pressure to the brake rotor and create friction established by the application of the brake pads to the rotor.

The power brake booster also helps to provide the force needed for the right amount of pressure to create an efficient application of force. It works by drawing power from a vacuum that is created by the engine during operation. This is why power brakes only work while the engine is started. The vacuum powers an internal bladder, which supplies the force to the hydraulic brake lines. If the vacuum is leaking, damaged, or the internal components within the power brake booster are damaged, it will not work correctly.

3 Methods for Testing Power Brake Booster Failure

Method 1: Testing a power brake booster is a fairly simple process. If you suspect the power brake booster is the root source of your brake system failure, complete the following three steps:

1. With your engine turned off, pump the brake pedal several times. This will ensure that there is no vacuum remaining inside the brake booster.

2. Press the brake pedal down hard one final time and leave your foot on the brake pedal as you start the engine. Do not release your foot from the brake pedal during this process.

3. If the brake booster is working correctly, you’ll feel a slight give in the pedal as the engine turns over. This is because the vacuum in the engine is supplying pressure to the brake booster.

Method 2:If you complete this step and the brake pedal does not move, it is an indicator that the power brake booster is not receiving vacuum pressure. This is when you should attempt a secondary power brake booster test.

1. Let the engine run for a few minutes.

2. Turn the engine off, then repeatedly press the brake pedal slowly. When you pump it the first time the pedal should be very ‘low’— meaning not much pressure resistance. As you pump the pedal, the pressure should become firmer, which will indicate that the brake booster is not leaking.

Method 3:If each of these tests passes, there are two more components you can check:

1. Inspect the Booster Check Valve: The check valve is located on the power brake booster itself. To locate it, consult your vehicle repair manual. You’ll want to disconnect the vacuum hose as it connects to the engine’s intake manifold. Make sure to disconnect it on the manifold and not the brake booster. If it’s working correctly, air should not flow with pressure applied. If air flows in both directions or you can’t blow air through, the valve is damaged, and the power brake booster will require replacement.

2. Verify Vacuum: In order for a power brake booster to work, a minimum amount of pressure is required. You can test the vacuum and verify that there is a minimum of at least 18 inches of vacuum pressure and no vacuum leaks.

If you don’t feel comfortable completing these tests, it might be a good idea to have a professional mechanic come to your location to complete an onsite brake system inspection. It is not recommended to drive a vehicle to a repair facility if you are experiencing brake system problems, so a mobile mechanic visit is a smart and safe idea.

Your vehicle’s braking system is essential to maintaining your safety and that of other drivers on the road. While a fault in any one of its components can lead to partial or total brake failure, the brake master cylinder is the most important part. You can ensure yours is working correctly by testing how it’s functioning. So, taking into account disc brakes, drum brakes, electronic stability control and anti-lock braking systems, how do you test a brake master cylinder?

Signs That You Should Test Your Brake Master Cylinder

When you step on the brake pedal, the pushrod contacts the primary piston, generating hydraulic pressure. This pressure does two things: It moves the secondary piston and half of the brake system, usually one rear brake caliper or wheel cylinder and the opposite front brake caliper. The secondary piston generates hydraulic pressure in the other half of the brake system.

Because the brake master cylinder is so simple, it is exceptionally reliable. The main concern is leaks. External leaks are not always obvious because the rear seal is buried inside the vacuum booster. Symptoms such as a sinking or spongy brake pedal or dragging or pulling brakes could indicate an internal leak, though. In some cases, the brake warning light, check engine light or a warning message might require you to test the brake master cylinder.

There are two basic tests you can perform to verify proper operation of the brake master cylinder.

1. In the Car: With the brake system bled, pump the brakes a few times and hold. The brake pedal should be firm. If the brake pedal is spongy, this could indicate air remaining in the lines or a mechanical problem, such as a sticking brake caliper slider. Bleed the brake system again and verify the calipers are moving freely.

On the other hand, if the brake pedal slowly drops, this could indicate a leak. If the pedal drops more suddenly, this is a sure sign that pressure is escaping backwards through one of the internal seals.

2. On the Bench: You can perform a similar test when inspecting a new or remanufactured unit or after removing the existing unit. Start with a bench bleed to remove all air from the master cylinder, then remove the bench bleed fittings and block the ports with bolts — do not over-torque the bolts. Use a screwdriver to press and hold the plunger in the rear of the master cylinder. The plunger should be very firm, if not immovable, past a few millimeters. If the plunger keeps moving in, this indicates a fault of at least one of the internal seals.

Fix Brake Fluid Leaks Before They Cause an Accident

Brake fluid leaks of any kind will put you and others in danger, so any master cylinder fault is reason for replacement. Some brake master cylinders require transferring the reservoir from the old unit. To install this, first bench bleed and check for internal and external leaks. After installation, fully bleed the brake system, making sure the brake fluid does not fall below the low mark. There may be special procedures for vehicles equipped with electric parking brakes, electronic stability control or anti-lock braking.

Check out all the brake system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on brake master cylinders, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

Are you facing a problem with the car brakes and finding it difficult, how to tell if brake booster or master cylinder is bad? Then, without wasting much of the time, look for the following symptoms in the brake system and follow the immediate maintenance tips for the respective part, which is running badly.

Symptoms: How to tell if Brake Booster or Master Cylinder is Bad

Brake Booster connecting the brake pedal and the master cylinder, is known to suppress the high fluid pressure by the use of vacuum stored in it. In fact, with this booster, the driver doesn’t have to exert extra pressure on the pedal, while stopping or reducing the speed of the vehicle.

Contrarily, the master cylinder found in every modern car today begins to operate when the brake pedal is pressed. It pushes the fluid from the reservoirs to the lines in the braking system, which exerts pressure on the brakes. And finally, this pressure slows down the car or causes it to halt.

Now, after comprehending the basic meaning and functioning of these components, let’s learn how to tell if brake booster or master cylinder is bad?

1. Symptoms of a Faulty Brake Booster

Vehicle Stops at a Longer Distance

When the air bubbles enter the brake lines via the master cylinder, it reduces the pressure that causes the brake to apply very softly. This situation immediately calls for the inspection of the valve (responsible for removing the excess air bubbles inside the system). Or else, the problem of vehicle stopping at a longer distance, after the brake is applied, would continue.

      SEE MORE:

• How to Bleed Brakes for Cars
• What Drivers don’t Know About Brakes System

Needs to Apply More Force on the Brake Pedal

The check valve is responsible for controlling the pressure inside the master cylinder. If this valve starts creating a problem then, it turns the soft and smooth pedal into the aggressive and hard brake pedal. So, whenever it gets difficult to engage the brake pedal, seek its inspection from the certified mechanic.

Stalling the Engine

When the diaphragm inside the brake booster fails, it allows the excess vacuum to enter the system from the engine. As a result, when the brake is pressed, the engine stalls, which can later cause issues that, are more serious.

Therefore, before you encounter any event of a brake failure, it is better to take your vehicle to the nearest trained professional.

2. Symptoms of a Bad Master Cylinder

Abnormal Clutch Pedal Behavior

If you have to exert extra pressure on the brake pedal then, there are chances that the master cylinder has the air inside them, which is further causing leakage of fluid.

Or, if the brake pedal is depressing itself then, this means that the heated brake fluid is not able to expand. Because of which, it causes extra pressure on the brake lines.

Worn Out Master Cylinder Seals

The rubber seal on the master cylinder wears out with the time. And, if these seals are not replaced timely, they may contaminate the brake fluid. Thus, causing the pedal to get soft and doughy or slowly sinking to the floor.

After knowing how to tell if brake booster or master cylinder is bad, it is the responsibility of the driver to keep a check on the brake system of the car. And in case, you locate any of the above symptoms with your car, it’s better to take it to the expert mechanics.