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Thread: Dual circuit brake tower query

  1. #1
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    Dual circuit brake tower query

    Hello from Brisbane.

    I asked much this same question some time before but don’t recall getting much of a response. It’s still relevant as I’m presently working on the brake tower and the answer would be useful.

    Here goes.

    The parts manual shows two different plates at the top of the pedal tower depending on whether the truck has single or dual circuit brakes. The plate for the dual circuit model appears to have two holes - one for the brake light activator switch and other seems to have a blanking plug. The part numbers for the activating lever under the switch is different as well. One other difference appears to be the hole for the activator switch on the plate with two holes is not centred which presumably explains why the activating plate is different. But the role of the second hole is unclear - why go to all that trouble to just fit a plug?

    Anyway, I have the plate from a truck with the single line system. So it has just the one threaded hole for the activator switch. I’m fitting a dual circuit system but will likely abandon the plate mounted activator switch in favour of a Series 2 type switch attached directly to the pedal inside the foot box.

    So, the question(s) is - why were the top plates and activating levers for the two systems different and can I just plug the existing single hole on the present plate?

    Cheers,

    Neil

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    Hi Neil.
    will take some pics, and get back to you.
    My boosted peddle is totally different, from single non boosted system, but even then, switch was a no go.
    Must be a third Type.

    whitehillbilly

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    JDNSW's Avatar
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    I thought all the dual circuit brakes had a completely different pedestal - the single circuit ones have the pivot between the footpad and the master cylinder, with the cylinder behind the pedestal, where the dual ones have the master cylinder between the footpad and the pivot, with the cylinder (and booster if fitted, which it usually is on dual circuits) forward of the pedestal.
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

  4. #4
    Johnno_p Guest
    This was the setup on my S3 ex army GS.
    The switch for the brake lights was inside.



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    Quote Originally Posted by Johnno_p View Post
    This was the setup on my S3 ex army GS.
    The switch for the brake lights was inside.


    Hello and thanks.

    That’s the plate with the two threaded holes that I was asking about. The plate on the tower that I am using, which also came off an ex-military 109”, only has the single threaded hole for the brake light switch.

    The parts manual shows these different top plates as respectively belonging to models with dual or single circuit brakes. Going back to my earlier question - what is the purpose of the second hole nearest the firewall? The manual shows a threaded plug and the plate in the photo seems to have an unused hole.

    Can’t see why they bothered to make them different - but engineers usually have a good reason. More important, does it really matter if I just keep the single circuit top plate anyway?
    Especially since I wasn’t planning to keep the brake light activator switch on the plate anyway.

    Cheers,

    Neil
    1975 S3 88" - Ratel

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    Quote Originally Posted by whitehillbilly64 View Post
    Hi Neil.
    will take some pics, and get back to you.
    My boosted peddle is totally different, from single non boosted system, but even then, switch was a no go.
    Must be a third Type.

    whitehillbilly
    Hello and thanks.

    My question was really about the top plate on the pedal tower of boosted brake system vehicles. I’m aware that the towers are different on the non-boosted brake models.

    Cheers,

    Neil
    1975 S3 88" - Ratel

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    Quote Originally Posted by JDNSW View Post
    I thought all the dual circuit brakes had a completely different pedestal - the single circuit ones have the pivot between the footpad and the master cylinder, with the cylinder behind the pedestal, where the dual ones have the master cylinder between the footpad and the pivot, with the cylinder (and booster if fitted, which it usually is on dual circuits) forward of the pedestal.
    Hello and thanks.

    It’s my understanding that the the towers, pedals etc are the same for either single or dual circuit brakes with a vacuum booster - master cylinder is obviously different. The manual just shows the top plate with the brake light activator as being different between the two models.

    Cheers,

    Neil
    1975 S3 88" - Ratel

  8. #8
    JDNSW's Avatar
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    To clarify - yes, the difference with the pedestal is between booster/no booster. But all dual circuit have boosters, and booster without dual circuit are, in my experience, very rare, at least in this country.

    To answer your original question - I do not know the reason for the difference, but I suspect it is because the expected pedal free play is different, meaning the switch has to be in a different location.
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

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