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Technical brake system question and issues

Discussion in 'Traditional Hot Rods' started by 34Joe, Dec 5, 2018.

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  1. 34Joe
    Joined: Nov 27, 2010
    Posts: 120

    34Joe
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    I have a question regarding the brake system in my 34 ford coupe. I have gm intermediate disk brakes up front and 11 inch ford drum brakes in the rear. The brake pedal is floor mounted with a 7 inch single diaphragm brake booster and a corvette style disk/disk type master cylinder( 1 inch).My problem is that I am unable to lock up the brakes. I have a 2 lb residual valve going to the front brakes and a 10 lb valve in the rear brake line. I also have an adjustable proportioning valve in the rear brake line. I have bench bled the master cylinder and power bled the system. I also need to know if on a disk/disk master cylinder does it make any difference which line coming off the master goes to the front or rear brakes would appreciate any info.
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 40,573

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    it should not make any difference which side of the master cylinder you use for which end of the car. With a disc/drum m/c, you need to use the large reservoir on the disc end of the car.

    What is the pedal ratio?
     
  3. V8 Bob
    Joined: Feb 6, 2007
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    Jim is correct about the disc/disc master, as long as it's 50/50 split. Most aftermarket are, but some OE are not, so know what you have.
    My experience with the 7" single diaphragm booster is not good. They will go to booster run-out quickly resulting in no power assist, and a low vacuum engine makes matters worse. A dual 7" or 8" is a much better choice.
     
  4. trollst
    Joined: Jan 27, 2012
    Posts: 1,241

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    I'll assume everything is new, the pads may need to be put through a few hot-cold cycles before they get some serious grip. However, does it slide in the gravel? Are you sure they don't lock up? Maybe you're not pushing yourself through the back of the seat, how does the pedal feel?
    Lots of variables here, can you be more specific?
     
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  5. 34Joe
    Joined: Nov 27, 2010
    Posts: 120

    34Joe
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    Appreciate the info,Thanks
     
  6. 34Joe
    Joined: Nov 27, 2010
    Posts: 120

    34Joe
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    It is an aftermarket master cylinder and as for using a dual 7" I just don't have the room, having an early TCI chassis with a tubular center crossmember. ..Appreciate the info. Thanks
     
  7. BJR
    Joined: Mar 11, 2005
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    BJR
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    from Minnesota

    Have you tried adjusting the proportioning valve?
     
  8. 34Joe
    Joined: Nov 27, 2010
    Posts: 120

    34Joe
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    Have tried adjusting the proportioning valve ,made no difference. ..Have it adjusted almost completely open at this point. appreciate the info, Thanks .
     
  9. Happydaze
    Joined: Aug 21, 2009
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    x2 What's the pedal ratio?

    Depending on that you might be needing a step down in m/c diameter to get the line pressure up.

    Regarding the 'bedding in' and heat recycling, it's possible to glaze the discs with too much low pressure activation such that they just don't bite. I managed to do that on a car that was driving and stopping kinda ok, or so I thought, but hauling it to a stop at just over 100mph at the end of a quarter mile really highlighted this issue and the cars braking was thereafter worse! Pedal ratio adjustment, 8" double booster and m/c change fixed that and I could then reliably almost stand it on its nose!

    Chris
     
  10. BJR
    Joined: Mar 11, 2005
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    BJR
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    from Minnesota

    Are you sure the residual pressure valves are put in facing the correct direction?
     
  11. 34Joe
    Joined: Nov 27, 2010
    Posts: 120

    34Joe
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    brake pedal ratio is 4 to 1. Appreciate the info, Thanks.
     
  12. 34Joe
    Joined: Nov 27, 2010
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    Have approximately 10K on brake system. Brake pedal has a solid feel. No matter how hard I push, the brakes don't lock up.
     
  13. 34Joe
    Joined: Nov 27, 2010
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    The pedal ratio is 4 to 1..
     
  14. Happydaze
    Joined: Aug 21, 2009
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    4:1 seems a bit on the low side and is going to restrict the pressure output with a given pedal input, and as already mentioned the 7" booster doesn't really add much. You might be able to get a little more by re-drilling the pedal where the pushrod connects. Thereafter an eighth smaller on the master would help but pedal travel would be longer . This presumes this is the issue and it's not something else.
    Chris
     
  15. Jokester
    Joined: Jan 29, 2005
    Posts: 230

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    Don't want to hijack the thread, but what would be a recommended pedal ratio?
     
  16. JOECOOL
    Joined: Jan 13, 2004
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    I always used 6x1 in our race cars , no power booster
     
  17. 34Joe
    Joined: Nov 27, 2010
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    As per Master Power Brakes, 4 to 1 is an ideal pedal ratio when using a power brake booster. 5 to 1 and 6 to one ratios are used when the brakes are manual. Appreciate the info. Thanks.
     
  18. Happydaze
    Joined: Aug 21, 2009
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    But a 7" booster is pretty ineffectual in comparison with larger sizes so you've arguably got more or less got manual brakes! There's a chart somewhere which if you reverse engineer the maths tells you this. Here you go!

    http://www.classicperform.com/PDFs/BoosterBrakePressureChart.pdf

    and they're making their comparisons across the board with a 7:1 ratio. just saying.
    Chris
     
  19. Happydaze
    Joined: Aug 21, 2009
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    My calc based on 1" master, 4:1 pedal, 100lbs input and 50% boost gives a very modest just over 750psi which I believes explains your problem.
    A 15/16" master will bump that to 870 and a 7/8" to just shy of 1000. With the 7/8 and the pedal ratio tweeked to 4.5 and you're at 1122, which is hugely more than you have. I have a figure of 1000 to 1200 in my mind as being what's required, but happy to be wrong.
    But you have to have enough pedal travel to get that!

    Chris
     
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  20. squirrel
    Joined: Sep 23, 2004
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    do the math, save the world
     
  21. bchctybob
    Joined: Sep 18, 2011
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    My ‘31 sedan had the same problem and the same booster. I used an adapter from Speedway to do away with the booster, installed the (SSBC) recommended disc/drum M/C with a SSBC prop valve in the rear and it all worked great after some adjustment. I have never seen the need for power brakes on old Fords.


    Sent from my iPhone using The H.A.M.B. mobile app
     
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  22. 34Joe
    Joined: Nov 27, 2010
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    34Joe
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  23. 34Joe
    Joined: Nov 27, 2010
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    34Joe
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    I don't see any need for a power booster either, Do you have a speedway part number for the adapter, don't see anything like it in their catalogue. Appreciate any info Thanks.
     
  24. bchctybob
    Joined: Sep 18, 2011
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    Hey Joe, I used a Speedway p/n 910-31402. It’s actually designed to ADD a booster to a non-boosted system but I used it to do the opposite.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  25. 34Joe
    Joined: Nov 27, 2010
    Posts: 120

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    Thanks for the part number,, Joe.
     
  26. fiftyv8
    Joined: Mar 11, 2007
    Posts: 4,479

    fiftyv8
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    from CO & WA

    Great discussion, I also suffer similar brake characteristics mentioned in my Ford F100 pickup.
    I added an early Camaro booster and M/C (with the GM fixed proportioning block) to my existing brake pedal so ratio you would think would not be the problem unless I am missing something.
    I got Dakota disc up from and big early Galaxy drum out back.
     

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