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  •  » Overly sensitive power brakes

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    5/23/2013 9:07 AM  #1

    LondonSteve
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    From: London, Ontario, Canada
    Registered: 4/01/2013
    Posts: 136

    Overly sensitive power brakes

    I recently purchased a 67 fastback with 4 piston kelsey hayes power disc brakes, & drum on the back.

    I believe the car to have been originally disc, and the previous owner added the power booster and replaced the master at the same time. All brakes are new with only a few hundred miles.

    Ever since the booster was added the brakes are wildly sensitive. You cannot be too aggresive or it seems to grab and snap your head forward with not too much pressure. He was never able to figure out the problem.

    I don't see the external "tube" style disc proportioning valve attached to the distribution block, and am wondering if this is the problem, however he says the car was always disc, just not power, and he did not have problems before, other than needing a lot of peddle pressure for manual brakes. He owned the car for over 30 years.

     Pictures of m/c, booster, and distribution block are below. Any thoughts???? Should I add an aftermarket adjustable valve? Not sure if this will address the problam though, since I thought it is supposed to control rear wheel lock up. Not what I am experiencing. Brakes will grab even at low speed with too much pedal pressure. It has the correct power brake pedal and is mounted in the correct pivit location. Is it possible the master is wrong? I notice it has an adapter on the front port.


    Last edited by LondonSteve (5/23/2013 9:30 AM)

     

    5/23/2013 10:01 AM  #2

    MS
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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,659

    Re: Overly sensitive power brakes

    He said the car always had discs, just not power....  Well, that is likely incorrect.  All Mustangs 67 or newer than had discs came with a power booster.  The fact you still have a drum brake differential pressure valve also is indicative the car originally had drum brakes.  You need to do two things...

    1.  Check the brake line that goes between the distribution block (diff pressure valve) to the rear axle to see if there is a factory proportioning valve somewhere in that line.  It could be near the dp valve or it could be near or on the rear axle housing.  It will be a cylinder about 1/2" in diameter and 3-4" long, with a steel brake line in each end.  If you do not find one, replace the differential pressure valve with a 70 style combination valve that has the dp valve PLUS a proportioning valve in it.  If there is one on your car, there is no need to install any other type of proportioning valve.  There is no need for an adjustable valve on your car that has a stock type system.  The valve is NOT the problem with the sensitive brakes, though.

    2. Since there is already evidence of shortcuts taken on your brake system, check to see if they installed a power brake pedal.  The pedal should measure 5" center-to-center from the pedal pivot tube to the pin where the booster connects. (like the picture below).  A manual brake pedal only measures 2" center to center and, as you can compute, a power brake pedal has a 3:1 lever where a manual pedal has a 6:1 ratio.  If you have the manual brake pedal in there, the booster thinks you are pressing the pedal twice as hard every time you touch it.  That will result in very touchy brakes.

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

    5/23/2013 12:49 PM  #3

    LondonSteve
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    From: London, Ontario, Canada
    Registered: 4/01/2013
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    Re: Overly sensitive power brakes

    The pedal was the first thing I checked before posting. It is a correct power pedal mounted in the correct upper pivit position. Also judging by the firewall, the car appears to have been power (disc?) originally and the booster disappeared prior to the former owner getting his hands on it 30+ years ago. Given your multiplier math, that would explain why he always had to struggle to push hard enough.

    I agree that the tube proportioning valve is missing, I noticed that right away as well, but the distribution block does appear to be correct for a 67 disc car with low fluid switch ( NPD part # 2B075-2A ). I was thinking an in line adjustable valve in conjunction with this block for ease of installation, so I don't have to pull the block and re-plumb all the lines. ( Also since replacement tube valves are not available. )

    However, that still does not address my sensitivity issue. The master looks correct based on photos of replacements for disc cars, but I can't be sure. I seem to remember you posting about certain masters having something modern inside them from some manufacturers and might have to be modified. Can you explain that again, and could this be my problem? I have read you refering to bore size as well in the past. I assume this can impact sensitivity. I don't know what I have other than just looking at the shape o f the master.

    Is it worth pulling the master and replacing it with a known "correct" new disc master, ( NPD 2140-4A ), as an attempt to solve the problem, or is that just fishing with new parts?

       •Thread Starter
     

    5/23/2013 6:00 PM  #4

    GPatrick
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    Registered: 4/01/2013
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    Re: Overly sensitive power brakes

    Given the odd history, could you have a 15/16 bore master set up for the manuals?  Perhaps a 1" bore for power might cut down on sensitivity.  I guess I would pull it to figure out what you have rather than guessing.  Maybe a 1-1/16" would work.  I don't know that I would trust anything that looks "stock" because it is currently installed since it is apparent that someone has been messing with it and you have no idea how creative they were.

     

    5/23/2013 9:42 PM  #5

    MS
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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,659

    Re: Overly sensitive power brakes

    1" bore is correct for your car.

    The booster itself can be the issue.  There is a valve inside the booster that can go bad, and it can cause excess travel and oversensitive power assist.  I should have mentioned this earlier.  If you verify the 1" bore MC, the booster needs to go.

    If there is no cigar-shaped proportioning valve in the rear brake line, then an adjustable valve would be very simple to install in the rear line, utilizing the existing dp valve.  That would be simpler than changing to the 70 type valve.  Good idea.  The bad part is figuring out if the adjustable valve is adjusted right or not.  I have heard of some 67 cars having the cigar prop valve on or near the rear axle.

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

    5/23/2013 10:28 PM  #6

    jollem
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    From: New Zealand
    Registered: 4/02/2013
    Posts: 48

    Re: Overly sensitive power brakes

    Had similar in 66 coupe. Old guy at brake shop found the rear shoes were"different size "to the drum diameter and the ends of the shoes were :"grabbing" instead of all the shoe area,did a skim and 80% better.
    This was after i went to disc power front end and they were pretty viscious.

    Last edited by jollem (5/23/2013 10:29 PM)

     

    5/24/2013 5:30 PM  #7

    Ron68
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    From: Mukilteo, Washington
    Registered: 4/01/2013
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    Re: Overly sensitive power brakes

    I can remember many years ago having to have your shoes "arced" to fit your drums when you put on new shoes. They had a machine that would grind the shoes to fit the arc of your drums so you would have full contact upon first use and wouldn't have to wear "em in.

    68 coupe - 351W, 4R70W, 9" 3.25 -- 65 convertible - 289 4v, C4, 8" 3.00
     

    11/01/2013 4:32 AM  #8

    jollem
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    From: New Zealand
    Registered: 4/02/2013
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    Re: Overly sensitive power brakes

    M/Steve did the 66 have the rear proportioning valve as well?

     

    11/01/2013 11:59 AM  #9

    MS
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    From: Canyon Lake, Texas
    Registered: 1/24/2013
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    Re: Overly sensitive power brakes

    jollem wrote:

    M/Steve did the 66 have the rear proportioning valve as well?

     66 Mustang disc brake cars did have a proportioning valve in the rear brake line, mounted on the inner fender below the MC.

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

     

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