scratching my head - brake booster troubleshooting

I was asking a question about brake booster in another similar thread but I think I should open a new thread with my troubleshooting and confusion.

I started noticing a strange noise coming from the firewall driver side lately. After I turn off engine, it was very noticeable and will last about 5-10 seconds. I suspect it is coming from the brake booster.

Then I noticed another thread about noise from brake booster. I went check the the vacuum hose, check valve etc. Check valve is fine. But when I tried to put suction at the vacuum port on the booster, it does NOT hold vacuum. I posted it (sort of a small thread hijack) at the other thread and was suggested the booster is bad when it does not hold vacuum. I bought a new piece of hose just to be very sure to rule out leaking hose. I connected the new hose to the vacuum port on the booster and suck air with my mouth. It was definitely not holding any vacuum. At that time, I concluded my booster is bad.

Then I went ahead and plugged the port (the one to connect to the brake booster) on the manifold and totally disabled the booster. I went for a test drive and it was a lot harder to brake (fully manual braking). I was very confused. If the booster doesn't hold vacuum, it will not give power assist. But when I disable it and I lose power assist. So, the brake booster seems working even if it does not hold vacuum.

I'm very confused at this time. Does the booster start holding vacuum when brake pedal is pressed ? or it should hold a vacuum regardless ? or I have a partially working booster ?

Thanks
- AC

Hey AC439. I am having the same issue about my booster but I made a conclusion that the booster should be replaced. Mine has a different story. My master cylinder leaked out and fluid found its way into the booster. That, I reckon, damaged internal booster parts (particularly the diaphragm), so it had to go. In your case, the vacuum from the intake manifold tube gets into the booster only when you press the pedal. A "gate system" (if you wanna call it that way) by that diaphragm allows vacuum into the chamber which then helps the driver stop the car with ease.
Hope that helps.

Quote:

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Originally Posted by AC439

I was asking a question about brake booster in another similar thread but I think I should open a new thread with my troubleshooting and confusion.

I started noticing a strange noise coming from the firewall driver side lately. After I turn off engine, it was very noticeable and will last about 5-10 seconds. I suspect it is coming from the brake booster.

Then I noticed another thread about noise from brake booster. I went check the the vacuum hose, check valve etc. Check valve is fine. But when I tried to put suction at the vacuum port on the booster, it does NOT hold vacuum. I posted it (sort of a small thread hijack) at the other thread and was suggested the booster is bad when it does not hold vacuum. I bought a new piece of hose just to be very sure to rule out leaking hose. I connected the new hose to the vacuum port on the booster and suck air with my mouth. It was definitely not holding any vacuum. At that time, I concluded my booster is bad.

Then I went ahead and plugged the port (the one to connect to the brake booster) on the manifold and totally disabled the booster. I went for a test drive and it was a lot harder to brake (fully manual braking). I was very confused. If the booster doesn't hold vacuum, it will not give power assist. But when I disable it and I lose power assist. So, the brake booster seems working even if it does not hold vacuum.

I'm very confused at this time. Does the booster start holding vacuum when brake pedal is pressed ? or it should hold a vacuum regardless ? or I have a partially working booster ?

Thanks
- AC

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I know you probably want another opinion other than mine but your booster is going bad. The dead giveaway here is that it doesn't hold a vacuum. It should hold the same vacuum for hours if it was good. Another thing is brake boosters don't just give out all at once, the seals inside wear out/dry out and they start to leak, so the assist they give you is not as great/good as it should be if all was working well but as you are noticing it is still giving some assist. Think of it this way, a good booster should have 25 in/hg of vacuum in it but because yours is leaking its only at 15 in/hg. (these numbers are made up as an example)

The booster holds a vacuum anytime the engine is on. This is why one of the test is to press the pedal then turn the engine on, the pedal should move down a bit. When the engine is on the vacuum is assisting you in pushing down the pedal.

Our booster works the same as all other boosters, you can google how to test or google signs of booster failures and how they work in general to get a better understanding.

Thanks both for replies. I talked to a mechanic friend of mine and he did look at the brake and booster. I will get back to him for vacuum test cause I don't have a vacuum pump. He said my redneck mouth suction test is not conclusive. But I would expect the booster will hold vacuum when I'm not touching the pedal. I'm also leaning to a bad booster but he said chances are slim. The sound the booster makes is like boiling water. But he is leaning towards the master cylinder.

I am not sure how easy to replace the booster but my friend said it is quite involved. I know there are bolts inside under the dash panel and wonder if it is easy to get to. I have replaced the MC before and it wasn't too bad.

My other concern is if the booster is not holding vacuum, it will affect the manifold vacuum and will in turn affects ignition timing (vacuum advance less now) then fuel efficiency etc.

Thanks.

Your friend is right, a booster will sound like boiling water but depends on how it fails. Most of the time boosters fail because brake fluid is leaking out of a bad and leaking master cylinder into the booster destroying the seals. This is liquid "brake fluid" can sound like boiling water. Consider also the age of these cars, after about 30 years the rubber seals can dry and crack causing a slight leak. You can only know for sure if you test with a vacuum pump.

Replacing the booster isn't terrible and can be done without removing much else that's not related to the brake booster.

You are right that the vacuum can cause the engine to run poorly but from your descriptions and the test you did were you plugged the booster vac line showed that the engine still ran as normal so if you have a leak its not that bad. You have to have a really big leak and terrible brakes for it to effect your engine that much.

Thanks for reply. I'm pretty sure the booster would have to be replaced if it doesn't hold a vacuum. Since my engine is FI, I think the vacuum leak into the manifold is compensated by the ECU so car still runs. However, I feel my low speed, low rpm acceleration is not as good as before probably due to lower than normal manifold vacuum. I did plug the port on the manifold when I tested with booster disabled. Seems like it ran a little better with it plugged but I only tested it around the neighborhood without power assisted braking. I shall start ordering parts (booster and MC). If booster is bad, I might as well replace the MC and bleed the brake lines.

I read somewhere that there is not much room to take the booster out until removing the windshield wiper motor.