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  1. #31
    Administrator blackpopracing's Avatar
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    A common question is ‘What bore size master cylinders should I use? In theory, it is possible to determine the correct size of the master cylinder (piston diameter) by calculating pedal ratio, pedal travel and caliper piston diameters but it’s often easier to ‘suck it and see’ by choosing a .750" master cylinder to begin with and working from there.

    Many factors will determine the optimum master cylinder size such as weight of the vehicle, tyre diameter, brake pedal length, weight distribution and servo or nonservo assistance. Brake pedal ratio is often the easiest parameter to change. Increasing the pedal length is, in effect, increasing the leverage but the pay-off is less fluid movement for a given pedal travel. If the pedal is 12" long from the fulcrum to the foot pad and the cylinder pushrod is 3 inches from the fulcrum then the pedal has a 3 to 1 ratio. Increasing the ratio to say 4 to 1 will give more leverage but with longer pedal travel.

    For a clutch the same rules apply to the release fork. A higher ratio will make the clutch action easier but there will be less travel of the release bearing. Determine the distance the release bearing has to travel to fully disengage the clutch (usually about 10mm) and work backwards from there.

    A release fork with 2 to 1 ratio will require 20mm travel of the slave cylinder.

    Brake hydraulics is really all about fluid movement so, to give you an idea of the effect of changing master cylinder sizes, here’s a chart of how much fluid each size master cylinder moves with each 1cm travel of the push rod. You can see that a 1" cylinder will move 2 ½ times more fluid than a .625" cylinder. But the pedal will require 2 ½ times more force to get the same braking performance. However, the pedal travel will be 2 ½ times less. Of course, if you want to increase braking power without changing the pedal or cylinder you could just add a servo.

    CYLINDER PISTON DIAMETER FLUID MOVED FOR 1cm OF PISTON TRAVEL

    0.625" ( ⅝") 1.98 cc

    0.750" (¾") 2.85 cc

    0.825" (⅞) 3.45 cc

    1" 5.06 cc

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  3. #32
    Rods 'n' Sods Junkie
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    No sorry, the fronts are m16 from (I think) mk2 escort rears are mk3 escort
    Ian

  4. #33
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    Quote Originally Posted by kapri View Post
    Mk 3 Escort fronts ? If so that will cause imbalance problems as they will be same size as M16 ( 54mm), need to be smaller like Mk3 Granadas at 38m bore You cam get a 7/8 and bigger in Lockheed as fitted to Landrovers same bolt pitch as Wilwood and Girling as well. Cheap as chips on ebay.
    I'm not sure now! The fronts are m16 (mk2 escort I think) rears are from a mk3 escort, will that cause an imbalance?
    Ian

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  6. #34
    Off the Xmas card list kapri's Avatar
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    The main question is have you used front calipers on the rear ? How have you set up the handbrake ?

  7. #35
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    No, rear calipers on the rear, stock handbrake cable

  8. #36
    I'm a grown up member now !
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    MickRich is offline
    have you plumbed in a proportioning valve to the rears also, a good chance you could have them locking up before the fronts only using a single circuit system otherwise

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