brake master cylinder
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  1. #1
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    brake master cylinder

    currently on the T Bucket there is a big old sherpa servo thats just taking too much room up so im looking at going with a new pedal with wilwood type master cylinder fastened to it to save on room,now i see there are a few bore sizes and ive read about measuring disk size etc but thought i would ask you lot here for some advice before i buy.
    its running disks all round,transit mk2 at front and volvo 240 at rear, what else do i need to take into consideration? car weight? performance?
    probably a stupid question but if i dont know then i dont know

    many thanks

    chris

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    I'm a grown up drunk! chevy2's Avatar
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    If the brakes work ok and you're gonna still use a servo then go for the same bore,
    its not the disc you need to consider but the amount of fluid reqd to move a particular caliper,
    ie, a big pistoned 4 pot caliper front and rear will need a bigger master to make it work,
    a smaller master may work but with fabulous brakes but a very low biting point,
    whereas a bigger master will have a higher biting point but with less braking efficienty.
    all about matching the amount of fluid to be moved in relation to caliper capacity,
    theres also brake ratio to consider to confuse it more.

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    i want to get shut of the servo, can i just use cylinder?

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    Off the Xmas card list kapri's Avatar
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    The servo increases line pressure if you take it off you'll have to push a lot harder ( up to the physical point where you can't anymore ) to get the brakes to work. Basically you need around 1500psi line pressure to stop an all disc system, 1000 psi as a minimum . We'll presume you have a 7/8 m/c with a 2:1 servo ratio giving you approx 1500 , take the servo off and you'll be down to 750 line pressure. If you want good line pressure without the servo you need TWIN master cylinders NOT twin circuit but actually 2 cylinders side by side with probably 5/8 m/cs ( without doing any calculations.
    Last edited by kapri; 17-01-2016 at 11:31.

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    I'm a grown up drunk! chevy2's Avatar
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    Of course,
    it'll obviously be more pedal effort,
    What calipers?
    the first master cyl you fit may be either too big or small to get the right balance of travel and pressure,
    1" bore is common on US style non-servo systems but thats based on a common combo of large single piston GM calipers and 9" Ford drum brakes.

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    Many problems occur when trying to use US proven tech on Brit / Euro sized systems or worse still a combo of the two For this set up twin 3/4 on a 6:1 pedal ratio would make around 1350 psi , dropping to 5/8 would push that up to around 1750. For systems based around Brit / euro components there is a fairly straightforward and well proven set ups . Problems also occur when people try to utilise US m/c servo mc combos based on price and size rather than suitability

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    Quote Originally Posted by chevy2 View Post
    Of course,
    it'll obviously be more pedal effort,
    What calipers?
    the first master cyl you fit may be either too big or small to get the right balance of travel and pressure,
    1" bore is common on US style non-servo systems but thats based on a common combo of large single piston GM calipers and 9" Ford drum brakes.
    Indeed but that would be too big for a Brit system were you'd want 7/8 m/c( possibly 15/16ths in certain circumstances ) for all disc set up . Generally 7/8 is for all drum on US stuff . Then you get a 1" Corvette /c for all drums but that is balanced against the wheel cylinder size which is HUGE compared to a Brit m/c . In general with Brit set up using drums on rear you use a m/c size corresponding to rear wheel cylinder bore . There's obvisly lots of variation depending on brake part combination but it has always appeared to me that brakes are always an afterthought, especially with m/c sizing as it's more often that not " What are you using?" or " What's small enough to fit this tiny little hole I've got left ?"

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    Off the Xmas card list kapri's Avatar
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    I've mate running a totally undersized m/c on an all disc set up that in theory shouldn't even be possible to bleed . I've known the car for best part of 40 years and it's ALWAYS had that system on it. However it has also ALWAYS had a nasty habit of losing it's brakes . I finally worked out why both happened and it's basically an accidental bodge that works as long as you keep on top of it . Because he doesn't understand brakes he won't change it because he thinks it's fine .

    Likewise another mate has another 30 odd year old rod and he had a mate go through it as the brakes had always been poor. After replacing all components I finally discovered it had a 2:1 pedal ratio !! No wonder they were poor . Just shows what most people will put up with " cos it's a hot rod "

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    THeres tons of tech listed above here and I wouldn't argue with any of it. starting with what you have and then moving forward would work for me. safe bit of road preferably straight and down hill. Start by discharging the servo. with engine off press on and off the brakes listening as depending on where servo is you can sometimes hear its slight hissy sound as it works, when discharged the pedal should get more solid and maybe feel higher up. or disconnect servo pipe and block it temp.
    roll down the hill and try stop the car noting how much pedal movement you have and how much effort it requires.

    don't know the exact spec of the car weight or brakes so cant even hazard a guess but you can usually find a bigger or smaller bore that will fit where your pedal is and that will exchange pedal weight for pedal movement smaller bore is lighter but requires more pedal movement bigger bore heavier with less movement (because you are moving more (or less) fluid per MM of pedal movement this can also be fettled by making the master cyl rod adjustable which can make sure the pedal to when the master cyl starts moving fluid has only a slight amount of freeplay.
    I hope you are lucky and the system will just work without the servo T SHOULD be lighter than the donor vehicles........... SHOULD being the optimum word!!!

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