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master cylinder bore size on 83 750 Maxim?

Discussion in 'XJ Technical Chat' started by fiater, Apr 22, 2008.

  1. fiater

    fiater New Member

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    Does anyone know the bore size on this master cylinder (factory)? I bought a no-name replacement on ebay and cannot get it to build pressure to save my life. Must have ran a gallon of Dot 3 through it, bought a rip off 30 dollar eze bleeder (literally a two dollar medical syringe) pushed fluid in from bleeders, and still no luck. My pads are both almost down to nothing, and I'm thinking the MC I bought (12.7mm) isn't big enough to build pressure on the dual disk setup (with low pads, the caliper pistons are pushed nearly all the way out, leaving lots of fluid volume behind it in the caliper bore).

    When I vice grip the hose six inches from the banjo bolt (on the MC), the lever is rock solid, but with no vice grip, it goes all the way to the grip, so I know the MC is good. Maybe just too small?

    I looked in the Haynes, but no specs on the factory MC bore.

    Thanks,

    Carl
     
  2. greg_in_london

    greg_in_london Member

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    A 1/2 inch bore MC will only operate a single disc. You need one with a 9/16" or 14mm. I'd give the same advice for any bike with 99.9% certainty, but can be 100% for XJs.

    In finding a replacement, the main thing to beware is that the lever will pull back all the way (even though it won't be able to after you've bled it) without fouling on the switch cluster - otherwise just about anything will do.

    I doubt you'll find anything with a bigger bore (bigger than 9/16"), but don't buy it if you find one. One that moves more fluid will feel totally wooden and will give you less mechanical advantage and therefore less powerful brakes with less feel.
     
  3. Danilo

    Danilo Member

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    5/8" is (was ?) the *standard* Yama Twin caliper Master Cyl Bore size
    Rally easy to find, it's the other sizes that are difficult.
    Usually has the size on the Reservoir casting
     
  4. fiater

    fiater New Member

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    Appreciate it guys. I'm gonna scrap the 12.7mm bore and try again with new pads as well
     
  5. Danilo

    Danilo Member

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    Hmm don't be too eager First ya gotta get New pads :lol:
    The syringe is brst used Sucking the Fluid out thru the bleeder.. and often the only way to get air out of the Master is to squeeze the Brake lever very quickly about 20 times.
    Also there is varied opinion on the size of the Master actually best suited for Calipers. Yama was infamous for fitting a Larger than clever Master. Resulting in the Typical Old Yama's wooden brake feel. Racers usually go to a Smaller (yes) master for better feel. There are charts with optomised ratios (caliper to Master bores) onna Net... Find them.. (yes.. it's a skill testing answer) they are V Educational.
    But you first MUST get fresh pads or yer just wasting time.
     
  6. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    It takes a LONG time to manually bleed brakes on these bikes if your system was emptied. A syringe or vacuum bleeder is the way to go, and make darned sure that the bleeder screws are clean and not jammed with gunk or you'll never bleed them even with a vacuum system!
     
  7. MBrew

    MBrew Member

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    I agree with Danilo and chacal. If you are able to build pressure with your line pinched off, it is highly unlikely that you have a master cylinder problem. You have either not gotten all of the air out or you have a leak down stream somewhere.

    This link has some good info on it. It is written for the Goldwing, but since we are talking about basic hydraulics, it applies to everything. The advice offered is very similar to that from Danilo. If you follow one of the links it will take you to another site that has a ratio chart like Danilo mentioned.

    Just for reference, my 650 Seca has a 5/8" Master cylinder installed for dual brakes. The diameter of the rotors vary on these bike, (the Seca 650 has one of the smallest) and I assume the wheel size does too. The ratio of caliper diameter to wheel diameter makes a difference in braking as well.

    Does anyone here know what the piston size is on a Seca 650 caliper? Mikes XS has a selection of master cylinders and I may experiment with using a smaller bore on my bike, but I'd like to calculate the ratios first and I don't want to disassemble them to measure.
     
  8. fiater

    fiater New Member

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    I did check the MC, and the bore is 5/8, which equates to 15.8mm, so the 12.7 was way too small. I ordered a rebuild kit for the MC and new pads. I had no signs of leaks anywhere, so I am hoping this does it. I'm missing good weather.

    Side note: I pull the auto cam chain tensioner cuz I was hearing rattles I couldn't pinpoint. It's a giant pain with carbs on, and takes interesting combos of wrenches/sockets to get it done. Even with 33k, the tensioner was spotless - day one clean and working perfectly. Even harder to put back on though. I've seen many threads on here about tensioners, and maybe this will help some avoid the headache of wondering if there auto tensioner is sticky. Unless the oil has been severely neglected, I would imagine there all fine (not much to go wrong there).

    Thanks for all the bore info - it's gone to good use.
     

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