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  •  » Hyper-sensitive brake problem

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    10/24/2016 12:38 PM  #1

    67StangMan
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    Registered: 9/27/2015
    Posts: 26

    Hyper-sensitive brake problem

    * 67 Mustang
    * Versailles disc rear, all factory parts.  E-brake IS adjusted properly. No "blue" rotors.  Wheel rotates without drag.
    * '70 Mustang disc brakes up front.
    * Dual diaphragm 9" booster.
    *  1 1/16" MC (1998 Windstar)

    Pedal is way too sensitive. Brakes work too good.  Pedal travels about 1.5" to no more than 2" and takes almost no foot pressure. Can lock up brakes way too easy.  Dangerous, actually.

    A smaller bore MC would bring more pedal travel but higher pressure, correct?

    What's the fix?

     

     

    10/24/2016 12:55 PM  #2

    TimC
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    From: Wheaton, Illinois
    Registered: 4/01/2013
    Posts: 878

    Re: Hyper-sensitive brake problem

    I think your issue relates to the 1 1/16" bore m/c.  The larger the diameter, the greater the volume of brake fluid that is moved for a given pedal travel distance.  The larger the volume of fluid moved, the sooner the brakes are applied.

    Do you have a prop valve?

     

    10/24/2016 2:09 PM  #3

    67StangMan
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    Re: Hyper-sensitive brake problem

    No prop valve.

       •Thread Starter
     

    10/25/2016 9:13 AM  #4

    Gaba
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    From: Van Alstyne Texas
    Registered: 1/15/2014
    Posts: 1,009

    Re: Hyper-sensitive brake problem

    ding ding ding ding .. you solved your issue. . I think. ;P

     The proportioning valve is there to help the rears not lock up too soon. I would think adding that and setting it right will help  

    If it aint broke, I have'nt tried to "Fix" it yet!
     

    10/25/2016 11:26 AM  #5

    Michael H.
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    From: Laurel, MS
    Registered: 4/01/2013
    Posts: 869

    Re: Hyper-sensitive brake problem

    Gaba wrote:

    ding ding ding ding .. you solved your issue. . I think. ;P

     The proportioning valve is there to help the rears not lock up too soon. I would think adding that and setting it right will help  

    That is definitely true on a disc/drum setup, but I think he should not need one, in theory, since he's running disc/disc.

    Last edited by Michael H. (10/25/2016 11:26 AM)

     

    10/25/2016 3:58 PM  #6

    MS
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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,373

    Re: Hyper-sensitive brake problem

    This is typical of overboosted condition caused by too large booster and/or incorrect pedal ratio.  A 67 Mustang power brake system consisted of a dual diaphragm 7.5" booster and a power-specific pedal with a 3:1 ratio. The manual brake pedal had approx 6:1 ratio.  So, if the booster is bigger by 50% AND a manual brake pedal was used, the pedal would be oversensitive by a factor of three.
    It could also be a defective booster internal valve. That can cause real easy pedal until WHAM full force is applied.
    Let's see some pics of your booster and the brake pedal. See my FAQ page about boosters and pedals.

    The comments about the oversized master cylinder piston size ( should be 1.0")  are not valid and would  cause a stiffer pedal with less braking pressure, not an easier pedal.

    It sounds like you have mismatched components and with no proportioning valve you are driving an accident looking for a place to occur.

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

    10/28/2016 3:01 PM  #7

    67StangMan
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    Registered: 9/27/2015
    Posts: 26

    Re: Hyper-sensitive brake problem

    [color=rgba(0, 0, 0, 0.870588)]Thanks for the advice. I really do appreciate it. So, I realized the info I gave was partially incorrect. Sorry for that.[/color]

    I'm using a Versailles brake distribution block which I believe has has a built in rear proportioning valve. It is not adjustable. I ran it previously without issue, so I don't think this is causing my sensitive pedal condition. It was designed by Ford for 4 wheel disc, on the Versailles of course. Also, I don't see where a prop valve would solve my sensitive pedal. Prop valves change the ratio of pressure between front and rear systems, but they don't reduce overall brake system pressure as seen out of the MC.

    [color=rgba(0, 0, 0, 0.870588)]I'm using a power brake pedal. It is the correct ratio. Good call out though.[/color]

    [color=rgba(0, 0, 0, 0.870588)]The booster is a GPS 8" dual diaphragm unit, not a 9". It is brand new. It replaced a factory 7.5" dual diaphragm unit. The GPS is .5" larger, but I would be surprised if that would make such a significant difference in pedal feel/force. That said, the sensitive pedal showed up along with the new booster. Perhaps the internal valve is bad? I'll call GPS to discuss.[/color]

    [color=rgba(0, 0, 0, 0.870588)]So, would switching from a 1 1/16" bore MC to a 1.0" bore MC bring more pedal travel and require more foot force? If so, that's what I'm looking for. Should I switch to a 1" bore MC?[/color]

    Last edited by 67StangMan (10/28/2016 3:02 PM)

       •Thread Starter
     

    10/28/2016 8:54 PM  #8

    MS
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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,373

    Re: Hyper-sensitive brake problem

    A 1" bore mc will produce more force with longer pedal travel, but less leg effort when compared to a mc with larger than 1".
    Assuming nothing is wrong with the brakes themselves, including oil contamination, I would suggest a booster issue. Make sure the booster output shaft is adjusted correctly.

    Just wondering... Why did you change from a factory booster to an aftermarket?

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

    10/31/2016 12:05 AM  #9

    67StangMan
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    Posts: 26

    Re: Hyper-sensitive brake problem

    The original booster failed.  It never did provide the pedal assist I wanted, so I decided to get a slightly larger sized unit - the GPS 8" dual diaphragm.

    I spoke with GPS about my sensitive pedal issue.  They do not think the booster is the issue.  GPS has requested pressure readings at each caliper.  So that's my next step.  I'll report progress here, for education sake.

       •Thread Starter
     

    10/31/2016 8:30 AM  #10

    ultrastang
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    From: Camden, Arkansas
    Registered: 4/01/2013
    Posts: 570

    Re: Hyper-sensitive brake problem

    Do you still have a manual brake pedal or did you swap to a (longer) brake pedal for power brakes that mounts to a pivot point higher up on the pedal support?

    If you did not change brake pedals, it likely has too much mechanical advantage (leverage) when coupled with a dual diaphragm booster.

     

    10/31/2016 5:28 PM  #11

    MS
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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,373

    Re: Hyper-sensitive brake problem

    My suggestion would be to use a factory Bendix booster with the correct power brake pedal and a 1" bore master cylinder. That eliminates the mismatched parts and all the guesswork that is built into your system.

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

     

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