Master Cylinder bleeding - rear port doesn't push fluid immediately?

Discussion in 'Suspension, Steering, Brake & Wheel Topics' started by ULTM8Z, Feb 3, 2018.

  1. ULTM8Z

    ULTM8Z Veteran Member

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    May 19, 2000
    Los Angeles
    So I bought a 67-76 Corvette master cylinder with 1" bore. While bench bleeding, I'm noticing the rear side of the MC doesn't start pushing fluid right away. The front pushes fluid immediately..

    On the rears, I don't start getting fluid movement until about half way along the piston stroke.

    I then put my old MC on and it's doing the same thing. WTF? Don't know why I didn't notice it before, but I think that explains my late pedal? If I don't get fluid going until mid-stroke on the piston, that would explain why the pedal seems to always engage late.

    The front brakes have been hooked up to the larger rear reservoir (it's one of those cast iron MC's with the large reservoir in the rear and small one up front).

    SHouldn't both ports push fluid at the same time upon pressing on the piston?
     
  2. 8pack

    8pack Veteran Member Lifetime Gold Member

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    Aug 29, 2007
    Pittsburgh, PA
    I don’t have an answer to your question directly, but when I was bench bleeding mine I the rear started releasing air after the front and I realllllly had to push on the piston to get the last little air bubble out.
     
  3. ULTM8Z

    ULTM8Z Veteran Member

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    Los Angeles
    Looking at all these images on Summit and Jegs for pre-plumbed power disc brake kits (4 wheel disc) it seems the front MC port always goes to the front of the proportioning valve regardless of the MC type.
     
  4. 71RS/SS396

    71RS/SS396 Veteran Member

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    Aug 26, 2008
    Wake Forest NC
    That's normal, you want the front brakes to engage before the rears since they do the majority of the stopping.
    The only oem masters that I've seen that reverse the ports (front port goes to the rear brakes) is the step bore/quick take-up masters, they started showing up on GM cars in the late 70's early 80's.
     
  5. ULTM8Z

    ULTM8Z Veteran Member

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    Ok thanks. Yeah this was an eye opener.
     
  6. dadgonebroke

    dadgonebroke Veteran Member

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    Jul 1, 2008
    Kansas
    I wonder if this is my issue with my mechanic replacing my MC. He thought the new MC might be bad because less fluid came from the rear port.?
     
  7. ULTM8Z

    ULTM8Z Veteran Member

    8,390
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    May 19, 2000
    Los Angeles
    It's not really less fluid... it's just that it doesn't start pushing fluid out until about 1/2 of the piston stroke. Kind of a "delayed reaction" type of thing.

    Any rate it'll be interesting to see how the system behaves now. Won't know until this weekend.

    I had instinctively hooked up the front brakes to the large reservoir at the time (thinking that this was the natural way to do it). But if the large reservoir is supposed to go to the front discs, why would Baer supply a MC in the kit that's essentially backwards?

    Seems like they would have supplied one with the larger reservoir on the front (if reservoir size is indeed a concern for disc brakes).

    This is the one they put in the kit...

    http://baer.com/GM-Cast-Iron-.9375-Bore-Master-Cylinder-6150012.html

    It's been a long time since I initially installed the kit and I don't have the instructions anymore (nor do I see it on their website). So I don't know what they said to do at the time. It could be that they said to hook the front brakes up to the front (smaller reservoir) and I missed it. Who knows...
     

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