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Hot Rods Brake system woes

Discussion in 'The Hokey Ass Message Board' started by junkyardgenius, Jun 7, 2021.

  1. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

    I have a firewall mounted 62 Chevy master cylinder. Running Ford rear drums and Wilwood dynalite calipers on the front. I have bled the brakes and have a good pedal but the brakes are not up to snuff can,t get them to lock up. No power booster no residual pressure valves. I have a proportioning valve but have not fiddled with it yet.
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 48,757

    squirrel
    Member

    What pedal assembly did you use? more specifically, what's the ratio of the distance from pivot to pad, to distance from pivot to pushrod?
     
  3. KJSR
    Joined: Mar 7, 2008
    Posts: 2,412

    KJSR
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    from Utah
    1. Utah HAMBers

    See attachment
     

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  4. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

    I made my own pedals. Pedal ratio is about 7 to 1. I can measure everything tomorrow.
     

  5. Doublepumper
    Joined: Jun 26, 2016
    Posts: 1,019

    Doublepumper
    Member

    If your ratio is close to 7 to 1, it should be fine. You may need a master with a smaller diameter piston. You would loose a bit of pedal height but, would have increased system pressure.
     
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  6. Garpo
    Joined: Jul 16, 2016
    Posts: 229

    Garpo

    Pretty much all o/e systems running discs use a booster. If no room for that, it come back to pedal ratio or cylinder size.
    Ball park to save a lot of maths - 1/8" smaller master cyl. bore will give about 10% more line pressure, and about 10%more pedal travel.
    Have seen effective systems that use a 1950s style remote booster connected to the fronts only.
     
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  7. So what’s the bore sizes ?
     
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  8. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,134

    Happydaze
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    Not wanting to be a smart ass but I'm afraid this isn't close, even for a ballpark :(.

    Line pressure comparisons need to be made from the area, not the diameter.

    A reduction from say 1" diameter to 7/8" is a reduction in area of approx 30% which is a significant change. Others look like this:-
    1.25 to 1.125 >>> 23%
    1.125 to 1.00 >>> 26%

    Changes of 1/16" diameter will give the ballpark 10% pressure changes.

    Chris
     
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  9. F-ONE
    Joined: Mar 27, 2008
    Posts: 2,767

    F-ONE
    Member
    from Alabama

    ^^^
    I assume you have one of those Chevy/GMC truck master/clutch combo that was popular in the early 60s. Ansen made accessories/adapters for these. "Ansen Clutch"....

    It's a single pot master. Single pot masters were not designed for disc brakes.

    Braking with all drums is accomplished by built in bias between the front and rear wheel cylinders. The front wheel cylinders are larger, the rears smaller.
    Now with discs all this "bias" is all screwed up. The calipers are gigantic compared to a wheel cylinder. This is why factory proportioning valves were necessary. This is why on the reservoir on "some" disc masters are larger for the front.
    Yes, some have made a single pot work with disc brakes....It's not ideal.

    My advice....
    Either go back to 4 wheel drums or get a master compatible with disc brakes.
     
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 48,757

    squirrel
    Member

    I don't think this is really accurate. Modern drum brakes are self energizing, while disc brakes are not. So, the rear brakes will tend to "grab" more than the front brakes. The proportioning valve limits pressure to the rear brakes to solve this problem.

    This is also not really accurate. The reason that there is a larger reservoir for disc brakes is that the fluid moves to the caliper, as the pads wear, and the piston moves further out of the caliper, to take up the slack. On drums, the wheel cylinder always returns to the same position, and the adjuster takes up the slack. The disc side needs a larger reservoir, so that it won't run dry before the pads wear out. (because some people never check brake fluid)

    I've run a small single pot master cylinder with front disc/rear drum brakes on a couple cars...it works, but I do have to keep track of brake fluid level more than I would with specific disc brake master cylinder.
     
  11. alphabet soup
    Joined: Jan 8, 2011
    Posts: 1,502

    alphabet soup
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    Ran into a similar situation at a friend's house. He had no pedal free play. Master cylinder piston wasn't coming back all the way. Gene.
     
  12. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

    Ok just had a measure up. The bore is 1 1/8” ( probably too big) from the pivot to the center of the pedal pad is 15 3/4” and from the pivot to the push rod is 2 3/4”. The pedal is very firm and does not travel very far.
     
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  13. squirrel
    Joined: Sep 23, 2004
    Posts: 48,757

    squirrel
    Member

    5.7 pedal ratio, which is borderline for manual brakes...but the m/c bore is pretty large. I'd go for a 1" bore master, should make a noticeable improvement.
     
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  14. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

    Might have to rethink the cylinder set up. Might go to 2 separate ones if I can get em in the recess and can find some that aren't plasticky
     

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  15. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,134

    Happydaze
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    Ah ah! I believe those can be sleeved for a smaller diameter. Don't know where you might go for that, it would presumably involve smaller piston and seals etc, but I'm sure someone else will know!

    Chris
     
  16. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

     
  17. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,020

    Elcohaulic
    Member

    Manual disc/drum I like 8:1 ratio. When converting to front disc, I find changing the rear wheel cyls to the front wheels cyls that came with the car..
     
  18. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,134

    Happydaze
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  19. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
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    from Kernow

  20. squirrel
    Joined: Sep 23, 2004
    Posts: 48,757

    squirrel
    Member

    Changing the pedal ratio would solve the problem, although getting enough pedal travel might be more difficult.
     
  21. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,070

    jimmy six
    Member

    I use Wilwood 4 piston front discs and 11” Ford rear drums. May not matter with a single MC but my Wilwood MC is 7/8” by their recommendation.

    If your rear Ford drum brakes came off a power brake car you need to replace the wheel cylinders with a smaller size.

    I DONOT have a power assist system, the pedal ratio is just over 5-1. I can slide all 4 wheels in an emergency stop.

    Everything I read that you lead to you needing an assist system. You can add a remote Bendix style which mounts on your left inner fender but I am not sure about using one in your situation.
     
  22. HotRodWorks
    Joined: Dec 6, 2005
    Posts: 326

    HotRodWorks
    Alliance Vendor

    Here is a comparison between master cylinder bore diameter and line pressure at the wheel cylinder for the same brake pedal effort.
    You can get an approximate 66% increase in braking effort by going to a 7/8" bore. However, your brake plunger will travel an extra 66% to displace the same amount of fluid. It's a trade off between pedal effort and pedal travel. The same principle applies if you change the pedal ratio. If you increase it, you get less effort but you'll have extra travel.

    BRAKE PRESSURE.JPG
     
    Last edited: Jun 9, 2021
  23. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,134

    Happydaze
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  24. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

    Looking into getting my cylinder sleeved down to 7/8".
     
  25. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,134

    Happydaze
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    Steady on! That's a big leap from 1 1/8"; you could easily run out of pedal travel. Best do the maths. If you need help send me a PM, not that I'm a mathematician but I can do ok!

    Chris
     
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  26. HotRodWorks
    Joined: Dec 6, 2005
    Posts: 326

    HotRodWorks
    Alliance Vendor

    If you sleeve to 7/8” bore, your pedal travel will increase 66%. Measure what it is now, then multiply by 1.66. That will give you your new travel. You will probably be ok. It’s like going from 2” to 3 3/8”. If it’s not traveling very far now, you will probably like the extra travel and better braking force.
     
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  27. junkyardgenius
    Joined: Dec 29, 2005
    Posts: 834

    junkyardgenius
    Member
    from Kernow

    Got plenty of room for pedal travel.
     
  28. HotRodWorks
    Joined: Dec 6, 2005
    Posts: 326

    HotRodWorks
    Alliance Vendor

    I fixed a 34 Ford with the same problem. The brake pedal was like stepping on a rock and the car wouldn't stop very well. I don't remember the sizes, but we reduced the master cylinder bore quite a bit and it solved the problem.
     
  29. blue 49
    Joined: Dec 24, 2006
    Posts: 1,386

    blue 49
    Member
    from Iowa

    I have less than ideal pedal ratio and have great brakes with not too much effort using a 15/16" bore master. 60's Chevelle calipers, I think, and '77 Granada rear drum brakes.

    Gary
     

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