Brake Servo Unit...Booster

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canada karl
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Joined: Tue Jan 06, 2009 3:50 pm

Brake Servo Unit...Booster

Post by canada karl »

Hey Mike I need a memory refresh on what caused brake fluid to leak into the brake booster? Was it a leaky diaphragm? Is there a fix for this problem as I imagine internal parts would be impossible to find?
1976 530i. BMW 59 Triumph TR3A(rolling resto). 67 Triumph TR4A(salvageable). 86 900S Winter car
canada karl
Posts: 924
Joined: Tue Jan 06, 2009 3:50 pm

Re: Brake Servo Unit...Booster

Post by canada karl »

Or is it motor oil that somehow finds it's way into the booster?
1976 530i. BMW 59 Triumph TR3A(rolling resto). 67 Triumph TR4A(salvageable). 86 900S Winter car
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Mike W.
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Re: Brake Servo Unit...Booster

Post by Mike W. »

It's a bad master cylinder. IIRC it's a double seal there on the master, but it does happen occasionally that brake fluid can be pulled from the master into the booster since it's usually under vacuum.

So you need to rebuild or replace your brake master cylinder, then try to drain and perhaps clean out the booster, I'm not sure if the diaphragm is brake fluid rated.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter
canada karl
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Re: Brake Servo Unit...Booster

Post by canada karl »

Thanks for the info. 8)
1976 530i. BMW 59 Triumph TR3A(rolling resto). 67 Triumph TR4A(salvageable). 86 900S Winter car
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REman
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Re: Brake Servo Unit...Booster

Post by REman »

My 2 cents...when you have the booster out it's a great time to inspect the steering u-joint and replace if needed, along with any firewall pedal bushings.
'81 528i... mfg date 10/80...owned since 1990
PatinaBeforePolish
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Re: Brake Servo Unit...Booster

Post by PatinaBeforePolish »

canada karl wrote: Sat Jun 12, 2021 2:28 pm Or is it motor oil that somehow finds it's way into the booster?
I replaced the master cylinder at 195k and found about an ounce of what seemed like engine oil in the booster canister. I think that over time the 1 way valve in the vacuum hose to the booster had deteriorated allowing booster vacuum to draw fumes from the intake back to the booster which accumulated and settled in the reservoir. I replaced the valve, hose and master at the time (which I'm sure were original to the car) but haven't had a need to open it back up so this is only speculation.
79 528i
canada karl
Posts: 924
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Re: Brake Servo Unit...Booster

Post by canada karl »

It does look like motor oil. Where did you buy the one way valve for the booster?
Reman...thanks for the tip.
1976 530i. BMW 59 Triumph TR3A(rolling resto). 67 Triumph TR4A(salvageable). 86 900S Winter car
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528i-1981
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Re: Brake Servo Unit...Booster

Post by 528i-1981 »

I think they're readily available, but I bought one from Faulkner.
https://www.faulknerbmwpartssource.com/
(oo=00=oo) Eric
1981 528i Manual
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