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Old 07-16-2004, 12:46 PM
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69 Pontiac Brake Booster Problem..

My booster passes the booster test-(e.g. Turn the motor off, depress brake pedal 5-6 times to drain off vacuum, restart with pedal depressed, pedal should sink-it does), but I have a vacuum leak between the MC and booster-(I can hear it hissing, and can plug it with a rag), and no power assist on my brakes. I recently replaced the MC, and noted that the gasket between the MC and booster was torn slightly, but since I dont know where to find a replacement gasket, I simply replaced it and hoped for the best-(the power assist worked before I replaced the MC). A mechanic told me that the booster is probably shot, even though I told him it passed the test, and he didn't know if the gasket in question could cause the kinds of symptoms I'm describing. Does anybody out there know if this could simply be a defective gasket causing this problem-(as I recall, it's a round semi-hard rubber gasket w/a hole in the middle the MC push rod goes thru- didn't look as though it generate a vacuum seal even if it weren't torn). And, very importantly, does anybody know where I can find a replacement gasket, or, fabricate one out of sheet rubber or something ? I've heard replacing these boosters is very expensive and a helluva lotta work...any help will be greatly appreciated.

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Old 07-19-2004, 04:03 PM
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Doesn't anybody know how to diagnose these things ?
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Old 07-19-2004, 05:26 PM
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Anytime I've ever had a booster that leaked vacuum when stepping on the brakes I have always replaced the booster. As far as I know the booster is a sealed unit. Most boosters are fairly easy to replace. Once you unbolt the MC and pull it away from the booster there are usually four nuts that have to be removed from inside the car as well as the rod to the brake pedal assembly. Once disconnected from the inside she should just pull away from the firewall in the engine compartment. Most boosters also come with directions for checking rod length. Give us time, sometimes posts just get missed or lost in the mix.


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Old 07-19-2004, 07:55 PM
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Boosters are not bad to replace, but they can sometimes be expensive. A universal joint for a socket can help with getting bolts out on some.

If your 'no power assist' symptom a high, hard pedal, with little travel, and extreme pressure needed for any braking, that could definitely be caused by a vacuum leak at the booster.

I have sealed a couple with RTV between booster and master. Anyway, they still had power assist at startup like yours, before the RTV, but the engine ran like it had a vacuum leak. I never drove one before sealing it or replacing it. If you shut off the engine with your foot on the brake, the pedal will probably climb back up due to the leak. This would be like having a bad check valve. The engine cant always keep up with the assist needed, especially with a low vacuum engine.
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Old 07-20-2004, 01:24 PM
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I'm a bit confused. Yesgo, are you saying that maybe sealing the gasket area w/rtv might fix the problem ? The unit is kind of acting like it sometimes provides some assist when braking, but at other times, not much if any. Yeah my symptoms are high pedal/hard braking-if that helps any..and again, anybody know where I might be able to find the gasket that fits between the MC and booster ?
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Old 07-20-2004, 07:41 PM
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I donn't know. It might fix it, or it might just be a band-aid, and show up later. Who knows, maybe the master will go bad later from vacuum being applied to it. The Honda I did it to is still working after 5 years, and the Jeep is going on a year. I have installed rebuilt boosters which sucked air there, and they had a gasket or o-ring to stop it.

If it was mine, I would apply a 1/8"-1/4" bead of high temp RTV around the master flange, and let it set a bit, install, and then let it dry and see what happens. You can always peel it off later. Just dont start up before it dries, and dont use a big glob-full... you don't want it inside the booster.

Last edited by yesgo; 07-20-2004 at 07:51 PM.
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Old 07-21-2004, 11:23 AM
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Sounds like it might be worth a try. Then I should just discard the damaged gasket, and rely totally on the RTV seal ? I'm a little confused how these boosters work. What keeps the vacuum in the booster unit-which is on both sides of the internal booster diaphram as I understand it, from sucking brake fluid out of the MC in a properly operating setup-are the rear seals on the MC supposed to do this ? BTW, I have a fresh rebuilt MC unit on this booster. Tnx for your help..
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Old 07-21-2004, 01:27 PM
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I really don't know their internals that well. Most have a seal of some sort around the pushrod. Some also have a gasket between the flange mount, and some have an o-ring on the master where it goes in the booster bore. Someone told me a master will go bad if vacuum is applied to it, so i've been checking rebuilt boosters before installing the master. So far, every one of them (3) have sucked past the pushrod. I figure if I didn't check it, I would never know, so I pretend I didn't, and just trust that it was rebuilt correctly, and I was told wrong. They all stop well so far.

You could look at it that way too. If the gasket hadn't been torn, wouyld there not be a problem, or would there be a hidden problem you may or may not ever have discovered? Either way, you would now be driving and stopping instead of typing and pondering. I think they're designed to suck air there, personally, and that's why there was a gasket. If you think RTV will work in conjunction with the damaged gasket, by all means use both.
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Old 07-22-2004, 11:59 AM
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www.howstuffworks.com

has a little tutorial on how boosters work, and shows vacuum on both sides of the internal diaphragm. Power assist is applied when the brake pedal opens a valve on the far side of the booster, allowing atmospheric pressure to push in assisted by the vacuum sucking on the other side-MC side-of the diaphragm. So it seems to me that a properly operating booster should be sucking vacuum thru the MC hole. I'm guessing that fluid will be sucked out of the MC if its rear seal goes bad. Does anybody out there know for a fact that if this is true ? Tnx for your help Yesgo. I'll give the sealer fix a try, but it'll take me a few days to get around to it. Let you know what happens when I do...
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