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Thread: Master cylinder size

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    Default Master cylinder size

    Going to install the Empi disc brake kit this weekend. I am not sure what size mastercylindet i have now. I understand 3/4 is the usual size. Should I go to the 11/16 bore cylinder while I am at it. Will that give me a little more braking also? I understand it will with drum brakes. Just wondering.

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    7000+ Posts Jack Klarich's Avatar
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    Quote Originally Posted by starrider1946 View Post
    Going to install the Empi disc brake kit this weekend. I am not sure what size mastercylindet i have now. I understand 3/4 is the usual size. Should I go to the 11/16 bore cylinder while I am at it. Will that give me a little more braking also? I understand it will with drum brakes. Just wondering.
    In theory you would think a bigger diameter would give more braking

    But NO

    You are pushing more fluid to get the brakes to move the same distance

    Bigger is NOT always better
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    Quote Originally Posted by Jack Klarich View Post
    In theory you would think a bigger diameter would give more braking

    But NO

    You are pushing more fluid to get the brakes to move the same distance

    Bigger is NOT always better
    Like a garden hose.....The smaller the diameter the greater the pressure at the nozzle...
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    A smaller master bore diameter will make more psi at the same pedal pressure.

    But.

    The pedal will move further.

    Basically use as small of diameter master as you have the pedal travel for.

    It all depends on the length and ratio of the pedal. And none of this one off stuff is the same as the other. So it is real hard for anyone to be able to tell you exactly what you need. But we can help you make the decisions.

    If you think you have a 3/4 bore then try it and see.

    It will work. Pedal pressure might be stiffer than you like. If you have the room for a longer pedal movement the reduce the bore size.

    As an example.

    I have a fairly large lever ratio and not the room for a long pedal travel. So mine works with a 22mm master. This is far far from typical but is what works for mine because of these reasons.

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    Default Master cylinder size

    I'll just put the disc brakes on and try it. Right now I have a very stiff pedal. Pedal moves very little. I doubt if the pedal moves 3/4" when I brake. It could move a little more if it needs to. Wish I knew what size master cylinder I have now. I will post after I get the disc installed. Thanks

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    300+ Posts LarryA's Avatar
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    being disc brakes operate with so little clearance, a large bore m/c is not needed, whereas with drums, out of round, grooved, and shoes set-up too loose,you can use up a lot of fluid just taking up the slack.

    Also with drums you have the self-energizing effect of the shoes, so less pedal effort is required.

    I would think you would want about 1/3 pedal travel for a normal to fast stop. Also good leg angle makes for less apparent effort. When you have every thing right, it will feel right. Willwood makes m/c's, pedals etc.

    take a look at their stuff....well good luck

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    Default Disc brakes

    Thanks for the info.

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