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Checking for a Bad Brake Booster

Updated on December 31, 2016
Automotive brake booster.
Automotive brake booster. | Source

Don't know if you have a bad brake booster?

Here are three common signs that your brake booster may have failed:

  • You need more effort to apply the brakes (stiff brake pedal).
  • The brake pedal doesn't return to its original position by itself.
  • The engine rpm goes down when you depress the brake pedal at idle.

Before you condemn the booster, though, you need to confirm that the device has failed. This simple guide briefly explains how the conventional, power brake vacuum booster (found in most gasoline engine vehicles) works, and shows you a series of simple tests you can apply at home in a few minutes to save some time and money in repairs.

The tests come in separate sections for an easier diagnostic. For most tests you won't need any tools to know that the booster has failed. But, if you still need further confirmation, you'll also find a series of tests that make use of a simple vacuum gage and a hand-held vacuum pump to confirm your findings.

First, let's see briefly how a common brake booster works to help you make sense of the simple tests you need to do.

Index
I. How the Brake Booster Works
II. How to Do a Simple Vacuum Hose and Brake Booster Check
My Engine Misfires When I Depress the Brake Pedal
III. Checking Engine Vacuum With a Vacuum Gage
IV. Checking the Brake Booster With a Hand Held Vacuum Pump
Vacuum brake booster configuration. The push rod connects the brake pedal to the brake master cylinder through the center of the brake booster, which multiplies foot pressure on the pedal.
Vacuum brake booster configuration. The push rod connects the brake pedal to the brake master cylinder through the center of the brake booster, which multiplies foot pressure on the pedal. | Source

I. How the Brake Booster Works

Basically, the brake booster in your car helps multiply the pressure you apply to the brake pedal when slowing down or stopping the vehicle. You can find the brake booster (a large canister) mounted on the driver side of the firewall inside the engine compartment.

The booster has a simple configuration. A flexible diaphragm divides the booster into a front (engine side) and a rear (driver side) chamber, providing a tight seal between the two. On the outside, a thick hose connects the booster front chamber to the intake manifold as a source of vacuum.

A push rod (aka power piston) runs through the center of the booster. On one end, the rod connects to the brake pedal and to the brake master cylinder at the other.

The brake master cylinder attaches to the front and center of the brake booster. On a conventional booster, at the center of the push rod, you'll find a normally open valve that allows vacuum to enter the rear chamber. Also, the rear of the push rod works as a normally closed valve to keep atmospheric pressure out of the rear chamber until you push down the brake pedal. Thus, when the brake pedal is at rest, both the front and back chambers have vacuum in them.

When you step on the brake pedal to slow down or stop the vehicle, you also push on the rear valve and center valves. So the rear valve opens, allowing atmospheric pressure to enter the rear chamber. At the same time, the center valve closes the diaphragm valve, blocking vacuum to enter the rear of the chamber. Then, atmospheric pressure and vacuum help you to push the rod against the master cylinder, which uses a hydraulic system to apply the brakes without much effort from your part.

NOTE: On some models, both sides of the diaphragm contain atmospheric pressure when the brake pedal is at rest. When you push on the brake pedal, vacuum forms at the front side of the booster.

Now that you know how the brake booster in your car operates, you can use this knowledge to troubleshoot the device using a series of simple tests, without and with tools.

A vacuum hose connects the intake manifold to the brake booster.
A vacuum hose connects the intake manifold to the brake booster. | Source

II. How to Do a Simple Vacuum Hose and Brake Booster Check

Before checking the booster, it's a good idea to inspect the vacuum hose, fittings and vacuum check valve. This is where most failures affecting brake booster operation occur.


Checking the booster vacuum hose:

1. First, apply the Emergency brake and open the hood.

2. Locate the brake booster mounted on the driver's side of the firewall inside the engine compartment.

3. Visually inspect the hose that connects the brake booster to the intake manifold.

4. Look for hardened spots, cracks, swollen or collapsed areas, holes or other type of damage; also, make sure the hose is properly connected (not loose); replace the hose if necessary.

5. Then, check the one-way valve that connects the vacuum hose to the brake booster for cracks, looseness or damage (some vehicle models use an in-line check valve between the brake booster and intake manifold). The valve should allow flow from the brake booster to the intake manifold to create vacuum.

6. The valve should allow flow from the brake booster to the intake manifold to create vacuum. So disconnect the hose at the intake manifold and blow through the hose. If air passes through, replace the booster check valve.

7. Check the manifold port for buildup (where the hose connects to the manifold).

8. Now check for signs of brake fluid leak between the brake booster and brake master cylinder. If you see a wet or darkened area going from the center, down to the bottom of the brake booster, most likely you have a brake fluid leak and fluid might've entered the brake booster chamber. Take your car to the shop for an inspection, if necessary. You may need to replace the brake master cylinder, and possibly the brake booster.

9. Start the engine and let it idle.

10. Spray soapy water along the vacuum hose, vacuum check valve, and intake manifold fitting. If you see bubbles and water being sucked at any spot, you've found a vacuum leak. Replace the hose, fitting or booster check valve as necessary.

11. Turn off the engine.

You can use the brake pedal to diagnose brake booster operation.
You can use the brake pedal to diagnose brake booster operation. | Source

Checking the brake booster:

If the booster vacuum hose and fittings are in good shape, it's time to move to the brake booster itself. A common and simple way to test the brake booster is by using the brake pedal.

1. Sit behind the steering wheel, set the transmission to Parking (automatic) or Neutral (manual), set the Emergency brakes, and start the engine. Let it idle for two minutes and then shut if off.

2. Pump the brake pedal at normal foot pressure four times and hold your foot on the pedal pressing down slightly on it.

3. Start the engine. As you start the engine, you should feel the brake pedal moving downward slightly, about an inch or less. Otherwise, you don't have enough vacuum in the brake booster. To locate the fault, do the Engine Vacuum and Brake Booster tests described in the following sections.

4. With the engine still idling, remove your foot from the brake pedal and turn off the engine.

5. Depress the brake pedal (using normal foot pressure) four times. If you notice the pedal rising after the second or third you depress it, the booster is more likely holding vacuum. Otherwise, a vacuum leak is affecting booster performance. To locate the problematic area, test engine vacuum and brake booster performance using a vacuum gage and a hand held vacuum pump as described in the next sections.

6. Start the engine and let it idle.

7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine.

The pedal should hold its position, if not, there's a leak in the brake booster, valve, vacuum hose or intake manifold. Check the vacuum hose, booster check valve, engine vacuum and brake booster as described in the following sections.

If you feel the brake pedal too hard while driving, and the vacuum hose and vacuum check valve work okay, most likely you need to replace the brake booster. Troubleshoot the brake booster using a hand held vacuum pump as described in the following section.

A brake booster leak can cause an engine misfire.
A brake booster leak can cause an engine misfire. | Source

My Engine Misfires When I Depress the Brake Pedal

An internal brake booster leak may manifest itself through an engine misfire.

If an internal valve or the diaphragm has failed, it may cause a leaned air/fuel ratio and a misfire as you step on the brake pedal, usually at idle.

If you know the vacuum hose, check valve and intake manifold fitting are in good condition, use this simple test to check whether the misfire comes from the brake booster:

1. Start the engine and let it idle.

2. Apply the parking brake.

3. Cover a portion of the hose with a rag.

4. Ask an assistant to depress the brake pedal.

5. Clamp off the hose with a pair of pliers using the rag as a protector to prevent damage to the hose.

If engine idle smooths out and the misfire stops, the brake booster is causing the misfire and needs to be replaced.

You can use a vacuum gage to verify engine vacuum level to the brake booster.
You can use a vacuum gage to verify engine vacuum level to the brake booster. | Source

III. Checking Engine Vacuum With a Vacuum Gage

Your brake booster may be doing its job. Still, you may have noticed through your tests or daily driving that there's something not quite right with it. And you may be right.

If your booster barely passed the previous tests (or didn't), check that your booster is receiving the right amount of vacuum. That's what you'll do here. For this test, you'll need a vacuum gage.

1. Disconnect the vacuum hose from the brake booster and reconnect it using a tee union so that you can connect a vacuum gage as well.

2. Start the engine.

3. Your gage should read between 16 and 21HG (inches of mercury); If you get less than 16HG, check for a vacuum leak at the hose, intake manifold (a gasket leak or crack at the manifold itself), your engine (valve(s), head gasket); or a possible restriction to vacuum either at the vacuum hose, intake manifold port, or exhaust system (catalytic converter).

IV. Checking the Brake Booster With a Hand Held Vacuum Pump

If your previous tests point to a failed brake booster but you still need to confirm your diagnostic, use a hand held vacuum pump. These tests are simple and only take a few minutes.

If you don't have a hand vacuum pump, you may rent one from your local auto parts store.

1. Let the engine idle for about 20 minutes to let it reach operating temperature.

2. Shut off the engine and unplug the vacuum hose from the vacuum check valve at the brake booster.

3. Connect the vacuum pump to the check valve using one of the hoses that come with the tool.

4. Then, apply 20HG of vacuum to the brake booster.

5. Wait for 5 minutes. The booster should hold vacuum without leaking; otherwise, replace it (assuming the vacuum check valve and mounting gasket are good).

6. Now, without disconnecting the pump, push down the brake pedal once. You should see vacuum drop by about 5 to 10HG. If booster vacuum remains at 20HG or drops to zero, replace the brake booster.

7. Apply vacuum to the booster with the pump to bring it back to 20HG.

8. Depress the brake pedal and hold it down for 30 seconds. You should see booster vacuum drop a little and then hold steady for the remaining of the 30 seconds. If vacuum drops considerably, replace the brake booster.

The troubleshooting procedures described here apply to the common vacuum brake booster, but configurations may differ in some respects. If you still feel that your tests seem inconclusive or that your booster comes with a different configuration, consult the repair manual for your particular vehicle make and model. The manual will show you what extra tests you should apply to your particular booster. You can buy an inexpensive, aftermarket repair manual at your local auto parts store or online. Buying the manual is a good investment because it comes with many troubleshooting procedures for many automotive problems, a maintenance schedule, and how to do those maintenance tasks.

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    • Dan Ferrell profile image
      Author

      Dan Ferrell 7 months ago

      Hi marciano,

      If there's a small leak, it's upsetting the mixture, and could cause it to loose power.

      Goodl luck

    • profile image

      marciano 7 months ago

      can the brake booster cause the car to drive slower even if you mash the gas panel?

    • Kingsley Iyoke profile image

      Kings 8 months ago

      Nice hub. Thanks for the info