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How to Check for a Bad Brake Booster

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Common Signs of a Failed Brake Booster

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. The guide will then explain a series of simple tests you can apply at home in a few minutes to save some time and money on 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 gauge 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.


I. How the Brake Booster Works

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

III. My Engine Misfires When I Depress the Brake Pedal

IV. Checking Engine Vacuum With a Vacuum Gauge

V. 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.

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's 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 on 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 on 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.

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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.

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.

How to Check 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 types 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 a vacuum. 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.

How to Check 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 park (automatic) or neutral (manual), set the emergency brakes, and start the engine. Let it idle for two minutes and then shut it 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 gauge and a handheld 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 handheld 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.

III. 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 gauge to verify engine vacuum level to the brake booster.

You can use a vacuum gauge to verify engine vacuum level to the brake booster.

IV. Checking Engine Vacuum With a Vacuum Gauge

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 gauge.

  1. Disconnect the vacuum hose from the brake booster and reconnect it using a tee union so that you can connect a vacuum gauge as well.
  2. Start the engine.
  3. Your gauge 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).

V. 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-held 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 five 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 30 seconds. If vacuum drops considerably, replace the brake booster.

DIY Troubleshooting Can Save You Time and Money

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.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: When I press the brake pedal, it is hard, and I hear air coming out of the pedal. What could be the problem?

Answer: Check the vacuum hose and valve that connects to the brake booster. If everything is connected right, check the operation of the valve and the booster itself.

Question: After I stop the engine, I hear a hissing sound by the brake booster but while driving the brakes are fine. Also when I’m coasting in neutral, the idle sits higher than usual, 1300-1400 rpm. Also, sometimes the revs go up and down. No leaks on the hose intake manifold side. Do you think the erratic engine behavior is because of the hissing in the brake booster?

Answer: It is possible. The hissing sound is a symptom of a vacuum leak. If you hear it by the brake booster, it might be the internal diaphragm, check valve or hose. I think this other post may help you:

Question: I have a '65 Ford Mustang and noticed a whistle from the brake booster area when the engine is running, and it stops as soon as I apply any pressure to the brake pedal. The brakes seem to work fine. What should I check?

Answer: The check valve or hose connected to it might be leaking vacuum. Look around that area under the hood. You may be able to locate the leak source.

Question: How can I tell if a brake booster or master cylinder is squeaking?

Answer: Check the brake lines at the brake master cylinder. If there are too stressed, they may rupture. Sometimes they become damaged from road debris. Although is sometimes hard to notice a "stressed" line, check anyway. Also, the noise may be coming from the pedal assembly itself. You may try spraying some WD 40 at the pivot points and see if this makes any difference. If the noise comes only with the engine running, have someone step on the brakes with the engine at idle (set the parking brakes and transmission in Park or Neutral). Using a length of hose can help you isolate the noise to the brake booster (vacuum noise in the line or inside the brake booster) or the master cylinder (internal or external leaking). Check the master cylinder and booster for signs of leaks. Hope this helps.

Question: My 2000 Chevy S10 is making a grinding noise when I press the brakes with a lot of effort. What could the problem be?

Answer: Check the rotors and calipers. It could be a cylinder not working properly. Make sure fluid is reaching the caliper -- a clogged brake hose can do this as well.

Question: I replaced the brake booster, but it's hard to push on the brakes. What do I do now?

Answer: Make sure vacuum to the booster is good. Check the valve and vacuum hose for leaks.

Question: I have a 1997 Jeep Wrangler, non-ABS. I changed the front brake pads, rotors, calipers, and brake lines; flushed brake lines with new DOT 3 fluid; changed master brake cylinder. I’ve bled the brake lines countless times using air compressor pump and old methods. No air in brake lines. With the engine on, the brake pedal hisses and is super spongy, but it always returns to original position. What do you think it is?

Answer: The master cylinder is probably leaking internally. Have someone listen to the master cylinder when you depress the brake pedal to locate the source of the hissing sound.

Question: I have a Nissan Xterra 2000, and every time I push my brakes, it makes a hissing noise and the brakes get harder to push. When I stop at a red light or just press on the brakes, the vehicle vibrates really hard and moves back and forth. Do you have any suggestions you can help me?

Answer: Check the vacuum hose that connects to the power booster, and also the check valve. Inspect the connection at both ends as well. If you can't find the leak there, check the booster.

Question: So I know my master cylinder was bad and I took it out. It has a torn rear seal. And now I'm thinking the booster may be bad as well. I had no pedal, with the air rushing out of the back of the booster rod. Is that normal? The valve and hose seem good.

Answer: If brake fluid leaked into the booster, it may have ruined the diaphragm, causing it to leak.

Question: I have a 2007 Toyota Yaris and I replaced the brakes, rotor, and calipers in front. I bled the brakes, and added DOT 3 fluid. I have brake tension with the car on and once the car starts the pedal goes to the floor without any tension. I'm clueless on what the issue with brakes could be. Tried everything and thought it would be the calipers. What else can I do?

Answer: If there are no evident leaks, the brake master cylinder may have an internal leak.

Question: Could the brake booster make my car not tun at all? When I disconnected the vacuum line to the booster it cranks, but when reconnected it doesn't. I have a code pending MAF, input high.

Answer: Check the MAF and then check for possible vacuum leaks.

Question: I installed a 4 wheel boosted disc brake kit on my 1970 Dodge Challenger. Pedal feels right under light braking. Under heavy braking, it's just too hard as if booster has stopped working. Done every test on the system (many times) but cannot resolve it. Any suggestions where to look?

Answer: Check to see if you need to do any adjustment for the pushrod to move as necessary through the booster to fully activate the master cylinder piston. If you have the repair manual for your model, it may tell you how to adjust it. This is usually the case when modification have been made. Other potential problems include a bad brake booster or vacuum line (check valve) or master cylinder issues.

Question: There is a hissing sound only when engine is off. But no sound when engine running. Everything is OK: coolant, temperature, etc. It's a Hyundai i20 1.2 petrol. No braking issues. The pedal comes to its normal position when off and comes down normally when pressed with engine on. What is the hissing sound from engine bay?

Answer: If the hissing sound seems to come from the brake booster area when you turn off the key, inspect the check valve, booster vacuum hose and connectors. There could be a vacuum leak.

Question: Why does my brake pedal move up and when I push the brake pedal?

Answer: If it feels hard to push the pedal, there could be a problem with the brake booster, caliper, or brake master cylinder. If you feel a slight vibration (up and down movement) there could be uneven wear on the rotors or drums.

This other post may help:

Question: I have a 2007 Chevy Silverado truck. When I push the brake pedal, it gets hard. Then going about 45/50 the truck seems to want to brake and starts to shake. When at a light it seems like the brake is on and doesn't want to go. What would cause this?

Answer: If all the brakes seem to activate, the problem might be with the brake master cylinder, probably bad seals. If you feel the problem is coming from one of the wheels, the piston might be dragging. Another problem could be with the booster acting on the master cylinder, or traction control issue.

Question: I only hear a loud noise from under the hood of my car when stopping. As long as I push the brake pedal, the noise continues. Once I let up, it stops. This only happens sometimes, and only at low speeds. I've checked the booster, and it seems OK. Is there any other test I can do?

Answer: Check the rotors and calipers. There might be some contact between the two, usually because of extreme wear.

Question: I bench bled and replaced the master cylinder and bled all the brake lines. When I start the truck, the brake pedal goes straight to the floor. What causes this?

Answer: Usually this is caused by insufficient fluid, a system leak, a push rod in need of adjustment, a faulty brake booster.

Question: In December I had to replace all of my brake lines due to rust and one cracked open completely. I drive a 2004 Chevy Tahoe. Anyway. Today I got into my car and the brake pedal went straight to the floor. And after I drove about a block, it never built up any pressure so I was sure it had sprung another leak. It had! So my question to you is why?

Answer: I think it might have to do with the material GM used on this and other similar models for the brake lines. You are not the only one with this problem. Many others have complained about this particular issue. I wouldn't be surprised if someone has gotten into an unfortunate accident because of it. However, after an NHTSA investigation, nothing conclusive came out from it. You may want to take a look at this article:

Question: I’ve been given a quote to replace the brake booster on my 2010 Infiniti FX35 of $2300.00 plus tax. This seems unreasonable to me given that the average cost according to a website I just read is between $160 to $590. What should I do given the apparently excessive quote I’ve received from my dealership?

Answer: Dealer costs are always going to be high priced. You can shop around and get quotes from reputable shops in your area. They can set you up with a quality brake booster replacement and reasonable labor costs.

Question: My brake booster is out on my '79 Scout. No one has one in stock. I have found one in a scrap yard. Is there any way to make sure it is good without installation?

Answer: Outside the vehicle, you can connect the vacuum hose from the in-vehicle booster to the booster you want to test, or get a longer but similar type of hose to connect to the booster being tested.

With the hose connected, idle the engine and then push the rod in with your hand, just as if you were applying the brakes. Then check how the rod recovers. It should be an almost immediate smooth back out movement; otherwise, the diaphragm is sticking.

Then turn off the engine, and disconnect the vacuum hose. Wait a couple of minutes and remove the check valve from the booster. You should hear a hissing sound when the valve is removed. That tells you the booster chamber is holding a vacuum.

Question: I have a 95 Toyota Camry with a new booster already installed. The car does fine for a while. It then suddenly the oil pressure bottoms out and the brakes start seizing together, almost stopping the car completely. Shut the car off, then pump the brakes a few times, crank it back up and it does fine for another 50 to 60 miles. Why would my Toyota Camry have sudden oil pressure problems?

Answer: Suddenly losing oil pressure is not normal and it may not be the oil pump. You may want to check the oil pressure switch-sender unit. The computer may be reacting to the signal.

Question: My 08 Ford Explorer is acting like its giving out of gas and stalling out. Why is this?

Answer: There could be a vacuum leak or a problem in the fuel system (fuel pump, fuel pressure regulator). But first, make sure you've replaced the fuel filter on schedule. You may be dealing with a clogging filter.

Question: My brake booster is hissing when I press the brakes. And then all of a sudden, when I BARELY press my brake, the brakes engage, the hissing gets worse, and the pedal becomes loose and goes down quickly. And is very slow to come back up. What causes this?

Answer: Check the vacuum hose that connects to the booster. Either the hose, the connection or valve might be leaking air.

Question: I recently replaced my brake booster with a large ”dual diaphragm” booster. Trouble is, I get an audible “hissing”, vacuum, sound from my foot pedal. It goes away as soon as I depress the brake pedal. Any ideas? The brakes seem to work fine.

Answer: Check the vacuum hose that connects to the booster. The problem can be with the hose or the valve. The sound may also come from a bad master cylinder leaking brake fluid into the booster. Hope this helps.

Question: If a brake booster is out, the will pedal go all the way to the floor?

Answer: You may be dealing with a leaky brake master cylinder or hydraulic system. Usually, the pedal goes to the floor after the vehicle stops. If the pedal goes to the floor before the brakes engage, it could be a mechanical problem in the brake assemblies. Take a look at this other post:

Check the brake pedal symptoms.

Question: When I pump the brakes in my 02 Mustang it builds pressure and seems to be air in the system but then I give it a moment, and it goes right to the floor. My master cylinder isn't losing fluid, but it goes down when I pump my brakes, and then the fluid goes right back up. I feel air coming out from where it connects to the booster. I also get air bubbles in the cylinder. What do you think?

Answer: Seems like there's a vacuum leak. It may be the check valve, the vacuum hose between the valve and intake or the booster itself.

Question: I have power drum brakes. With the engine running, as soon as you touch the pedal it drops about an inch to inch and a half. Then you have manual brakes only. But it actually pulls away from your foot as soon as you touch it. I tried pushing with my finger and watched the pedal move away. Any ideas?

Answer: Make sure there enough brake fluid in the system, and it's not contaminated. Check the brake pads/shoes. Also, the brake booster or check valve may be leaking vacuum.

Question: I have a VW Fox. When the car is idling and I press my brake pedal, I find that the idle goes up and down as if the car wants to switch off. Could I get some help?

Answer: Check the vacuum hose and valve that connects the brake booster to the intake manifold. There could be a vacuum leak. Make sure it's properly connected and not damaged.

Question: My wife's 2012 Cadillac SRX has a problem with sudden braking. The pedal goes to the floor and the braking distance is very long. Afterward, subsequent braking is fine. Does this sound like the Vacuum Brake Booster Pump?

Answer: This sort of intermittent brake pedal fade is usually caused by a faulty brake master cylinder. There could be an internal or real leak on the assembly. You need to have the fault properly diagnosed and fixed before driving the vehicle again.

Question: Can a brake booster cause white smoke to come out the tail pipe?

Answer: White smoke through the tailpipe is usually a sign of a coolant leak into the cylinders. This other post gives you an idea about this type of problem:

Question: When the car is off, the pedal is up; when I start the car the pedal goes down. The car will stop but low pedal pump it once it up a little more. New master cylinder. Is it the booster?

Answer: It seems like a vacuum leak, but make sure you've taken all the air out of the hydraulic system. If everything is good, then there could be a vacuum leak in the hose the connects to the booster, the check valve or the booster itself. Sometimes, you'll be able to hear a hissing sound where the leak is.

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