firm pedal after depressing brake multiple times

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  • downforce22
    No R3VLimiter
    • Aug 2009
    • 3189
    #1

    firm pedal after depressing brake multiple times

    The problem is my brake booster seems to be very slow at building vacuum. If I drive for a while and apply the brake it depresses like it should, soft and very easily, but if I let off and depress it again quickly my pedal gets firmer and harder to slow down. I suspect the booster itself is operating decently because I still get the original amount of boost on the brakes a little while later.

    The car has cross drilled rotors and I am replacing the pads with oem compound. I am prety sure it was autocrossed and does have a cam in it. My mechanic friend said the cam could cause overlap making the booster take longer than stock to build up vacuum.

    Are there any tests I can do to check if the booster is working properly vs having a vacuum leak?

    Could it be related to my master cylinder?

    Remember this is an iX but I do suspect the booster works the same way as a regular i booster despite being bigger.

    Thanks

    http://www.r3vlimited.com/board/showthread.php?t=179390&highlight=bOOSTER&page=2

    Higher than normal pedal pressure can also be the result of insufficient vacuum to the booster. That can result from intake leaks or a problem with the check valve or booster hose.
    The booster functions by applying engine vacuum to the engine side of the diaphram and atmospheric to the firewall side. If it were like this all the time then the brakes would always be engaged. The pedal actuates a valve that controls what gets to the firewall side of the diaphram: when you press on the pedal it's atmospheric, when you let off it's engine vacuum.
    318iS Track Rat :nice: www.drive4corners.com
    '86 325iX 3.1 Stroker Turbo '86 S38B36 325

    No one makes this car anymore. The government won't allow them, normal people won't buy them. So it's up to us: the freaks, the weirdos, the informed. To buy them, to appreciate them, and most importantly, to drive them.
    Tags: None
    • jlevie
      R3V OG
      • Nov 2006
      • 13530
      #2
      My first thoughts would be air in the brake lines or a failing master cylinder. In almost all cases a problem with the brake booster will result in a hard pedal.

      I'd suggest that you start by flushing the brake system using a pressure bleeder. Push a measured 250cc through each rear caliper and 150cc through each front caliper. If that doesn't cure the problem (or you don't get air out). Replace the master cylinder.
      The car makes it possible, but the driver makes it happen.
      Jim Levie, Huntsville, AL

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        • downforce22
          No R3VLimiter
          • Aug 2009
          • 3189
          #3
          replaced my brake pads and that helped considerably (mintex). I may have a small leak in the line from the intake manifold to the booster
          318iS Track Rat :nice: www.drive4corners.com
          '86 325iX 3.1 Stroker Turbo '86 S38B36 325

          No one makes this car anymore. The government won't allow them, normal people won't buy them. So it's up to us: the freaks, the weirdos, the informed. To buy them, to appreciate them, and most importantly, to drive them.

            Comment

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