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Brake Booster

Discussion in 'E34 (1989-1995)' started by bswihart, May 18, 2011.

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    bswihart

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    Is there a resource/location that discusses common problems with the brake booster. I suspect a bad booster. they require a large amount of force to bring the car to a stop. The car has 275K miles and I have no idea if this is the original part. Is there a resource that discusses possible repairs? A new one is $250 +. I have checked the vacuum lines and all are attached with no obvious cracks or holes.

    I pumped the brakes about ten times and held the pedal as I started the engine. The pedal did not move when the engine started, so I suspect that there is no assist or "boost" being provided. Any suggestions or experience?
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    MGarrison

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    Sounds like you should get a Bentley manual - my E30 manual says if the brake booster is bad, it is not serviceable and has to be replaced; I would assume the E34 booster is no different. You say 'they' require a large amount of force to stop, I assume you mean when you apply the brakes, it takes much more pressure (and effort) than the normal amount applied to the brake pedal to bring the car to a stop?

    You'll want to start by checking for vacuum leaks before jumping into replacing the booster. Do you have a mityvac? If you pull the vacuum line from the booster (careful, it could take some effort, and you might find the plastic parts fragile, due to age/heat/etc), you could presumably use something like a mityvac to test if the booster will hold vacuum. I don't know how much vacuum power the booster might tolerate, so I would figure a hand vacuum-pump like a mityvac would be preferable to something like a air-powered vacuum pump, where you can't control the amount of vacuum applied. I don't think you need too much vacuum anyway, enough for the gauge to show the vacuum and whether it's holding should be sufficient.

    But, that's not the be-all and end-all. There's a rubber ring-gasket on the brake master cylinder where it bolts to the vacuum booster. I wouldn't rate it as likely, but it's possible for a leak there. There's a one-way check valve in the the vacuum-line circuit to the booster, you want to make sure that's working. You'll want to check for vacuum leaks overall, that could be a problem. A Bentley manual could more handily explain the various possibilities for vacuum leaks. If you have to replace the booster, you'll probably have to remove the brake master cylinder. If it's original (or old), be careful on bleeding not to push the brake pedal any further than you normally do, it's _easy_ to kill the internal seals in an old master cyl. by pushing it to the floor, and then you have to replace that too. If you do have to replace the booster, and the master cyl. is old, it might be advisable to replace it anyway.

    There may be a parts number sticker on the booster, which might also show the year or month/year of manufacture. Priobably worthwhile to take a look-see at the rest of the brake system while you're at it; both the flexible and steel lines running to the front and rear brakes, visually inspect the calipers, pistons, etc.
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    mooseheadm5

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    What year and model car do you have?
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    bswihart

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    This is an 89 535.

    I bought some vac hose, clamps and the little rubber grommet that attaches to the brake booster housing and this cured the problem. Total cost about $11. Nice!:) A mechanic told me that the brake boosters almost never fail.

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