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Thread: Loss of brake boost

  1. #1
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    Loss of brake boost

    I am autocrossing an 04 M3. The brake system is healthy, fresh Ferodo pads, flushed systems, etc. ABS works as designed. BUT, after trail braking into some corners and flat footing the gas to the next element, there is a pattern of lack (or non-existent) brake boost. ................ Anyone else had this problem? Any known solutions? In daily driving there are no problems.

  2. #2
    dworthy's Avatar
    dworthy is offline Wagon meister :) BMW Tech Expert
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    You can check the 1-way valve to ensure it isn't clogged, or the lines going to the booster. The brake booster uses engine vacuum, so if you are always at a high RPM, there is a chance it doesn't have enough time to build before you hit the brakes again.

    Lastly would be a bad booster, does it hold pressure, or do you hear a hissing sound when the car is turned off?
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  3. #3
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    Quote Originally Posted by dworthy View Post
    You can check the 1-way valve to ensure it isn't clogged, or the lines going to the booster. The brake booster uses engine vacuum, so if you are always at a high RPM, there is a chance it doesn't have enough time to build before you hit the brakes again.

    Lastly would be a bad booster, does it hold pressure, or do you hear a hissing sound when the car is turned off?
    No "hissing".
    As far as I can tell, the 1-way valve is good. But, I plan on changing it anyway.
    When I roll the car off of my trailer, with the engine off there is power assist --- ONCE !!

    I spoke with Active Autowerke (source of all my tunes), and Karl (senior tuner and human BMW encyclopedia) says that this is a common issue. My co-driver and I may have to lift (a bit) before some turns to generate vacuum.

  4. #4
    dworthy's Avatar
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    You should get 2-3 pumps off the brake pedal before it losses all assist. So check this: After you drive the car, let is sit overnight. If the pressure bleeds off by morning, I would do the check valve first. If that doesn't fix it, get it smoke/pressure tested.
    Darin
    Current:
    16 220i Active Tourer Platinsilver MET (C08)/Dakota Black (PDSW), P7ACA, P7LDA, P7LHA, P9BDA, 6sp Manual - Wife's new toy!
    05 325it Electric Red(438)/Gray(N6TT) ZCW, ZSP 5sp Manual Back set cover, trunk mat, Euro Infra-Rot front windshield, and mud flaps! Mr. Wagon My new Winter car.
    05 M3 Imolarot II(405)/Gray(N5TT) ZCW, ZPP 6sp Manual C.F. Lip, CSL diffuser, SSK, Euro Infra-Rot front windshield and a trunk liner! Mr. Go_Fast Stored for the Winter
    Past:
    95 318is Montreal Blue Met (297)/Beige(K1SN) RIP, killed by an Idiot.
    84 M535i gray market Burgandy Rot Met/Black Leather Lowered by Intrax on bilies, poly everywhere, B&B cat back system, K&N, and a hitch! Da Beast - Still running w/400k+!
    91 316i euro Tizianrot/Gray cloth - E-36 w/M-40 RIP, but great on gas! Best was 38 MPG
    82 528i euro Saphire Blue Met/Blue Leather RIP
    79 525 euro Green/Green RIP
    79 318 Silver/Black - The first one that got it all started
    M-Flight Member

  5. #5
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    Check valve is already coming.

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