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Thread: disc brake conversion

  1. #1
    Senior Member MASTER BUILDER
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    disc brake conversion

    I bought a 70 chevelle from racing junk last year and I am just now putting it on the road. The question I have is the guy I got it from put disc brakes on the front (not power disc) The pedal is rock hard. Does the proportioning vavle needs changed? I'm not sure whats wrong.

  2. #2
    Senior Member RACING JUNKIE
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    Probably the wrong master cylinder.

    If the bore is too big the pedal will be hard as a rock.If they are manual brakes find a master cylinder with a bore no larger than 1". I just went through this last summer. A 1.125 bore MC made my pedal rock hard...going to a 1" bore cured it.

    Another just FYI ....drum brakes have risidual valves to keep the springs from forcing fluid back to the master cylinder, they must be removed when you go to disc. I think they are usually in the master cylinder itself but I think some had them in the distribution block...if they are left in they can make the calipers drag the rotors

  3. #3
    Senior Member MASTER BUILDER
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    Ok how do I go about measuring the bore? Is it were the rod pushes on the master cylinder?

  4. #4
    Senior Member RACING JUNKIE
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    I went on NAPA Auto Parts web site and started poking in old Cars at random until I found what I was looking for...their website lists the bore size. I was working on a Nova with Wilwood brakes but ended up using the master from a 68-72 corvette I think it was...It has a 1 inch bore...it was made for 4 wheel disc brakes but as long as you instal a risidual valve between it and the rear drums it will work...I didn't need one for some eason but have only ran it once so far.

  5. #5
    Senior Member MASTER BUILDER
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    ok I'll get to looking at that site

  6. #6
    Senior Member MASTER BUILDER
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    I thought the residual valve on drum brakes was to prevent fluid drain down. In other words, if I was told correctly, if that valve is not there, the first time you kit the brake pedal, the pedal wll go to the floor. 2nd time is ok.

  7. #7
    Senior Member MASTER BUILDER
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    I touch the pedal and it feels like a rock. And I got to stand on the pedal to get it to stop. Any suggestions??

  8. #8
    Senior Member RACING JUNKIE
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    Quote Originally Posted by jreiley
    I thought the residual valve on drum brakes was to prevent fluid drain down. In other words, if I was told correctly, if that valve is not there, the first time you kit the brake pedal, the pedal wll go to the floor. 2nd time is ok.
    yep....that's what they say. On my NOVA that isn't the case, but that is what it's for. I do have an adjustable proportioning valve so maybe that's got something to do with it.


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