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Thread: REAR BRAKE MASTER REBUILD washer / seal stack. R100S

  1. #1

    REAR BRAKE MASTER REBUILD washer / seal stack. R100S

    Took apart rear master cylinder awhile back , the usual suspect Reference sources iffy / incomplete at~best .

    The Brembo kit comes with a poster-sized instruction sheet written with faded 6-pt type and not one diagram fully matches the piston / seal stack here.
    Bravo ! but why should you let Brembo Stop ya ?

    Picture below sure LOOKS like the proper stack but before assembling , I'd prefer to get some practiced eyes on this ,
    as the rearmost keeper ring is not a circlip style, rather this is a 1-time drive-in parts ( no mulligans ) . Needs 21.5 mm (max) drift

    Can any one recall if this is correct assembly order & orientation ? TIA

    the drive-in ring is tapered, going to drive into bore w/ smaller ID headmost.
    The flat washer just looks like a back-up for the outermost seal.
    This outer seal is a step / taper , also planning on narrow OD first.
    The piston is a natural with Flared lips FLARE headmost ( as an expand-against-pressure seal, seals installed at MFG on new piston in kit)
    and the internal return spring is a secure fit only on one end of piston,
    finally the 'stove pipe hat' grommet is well fitted into spring , cushions the far (outlet) end of cylinder with a hole for fluid passage.

    The feed tube grommet is not in the rebuild kit nor BMW catalog, but I located a match under a Duc part number.
    Fortunately too, since it was the crappiest part on the original master. But don't let Brembo Stop ya.; put it in the kit, wont ya?

    So if you know the stack, please agree with the order & orientation below , or set me straight.
    Otherwise , take this diagram to guide your work & your very welcome. Then I can use the instructions as an emergency blanket.



    bmw r100s rear master seal orientation - 1.jpeg

    bmw r100s rear master seal orientation - 2.jpeg

  2. #2

    whats that dip doing now ? Epoxy-clad master class

    as a matter of fact , I am going to dip this master in clear epoxy before rebuilding,
    the master being a pretty good looking casting / alloy after a little polish,

    will be interesting to see how epoxy survives on a polished Al part , even with eau D'paint remover ( DOT 4 ) vapors working away on it.
    JB Weld used to offer an epoxy for master cylinders ,NLA, but I'll try a slow set / clear, so its not a totally whacky tacky idea.


    I heated-molded a plastic spice bottle as conforming tank / holster for dipping the master body to reduce the epoxy required.
    The line on the bottle is the minimum fluid level to fully cover while dipped, determined with water quicker than ol' archimedes can say " You Reek, hah ! ".
    The tail / hinge end of the master is covered with a dust boot, so coverage on that end is not too critical.

    Im using slightly thinned slow-set clear epoxy, thinned in hopes that the dipped part will drip-drain the excess before the catalyst kicks in .
    A thin layer is best, thicker layers reduce translucency. 15 mil coat is target.

    I undersatnd that grain alcohol is a de-natural as an epoxy thinner, non-degrading up to about 20~30% dilution,

    of course the epoxy is best limited to the exterior of the MC body,
    It so happens that a stub of garden hose fits snug into the tail end as both handle & seal ,
    and silicone plugs will cap the inlet/outlet ports.

    Soon as the epoxy starts to gel, I'll pluck the hose & plugs out to avoid an expensive piece of vintage bike art deco.
    The screw mounting hole are open to handle the part wet without ruining the finish, and razor trim any zits & drips when cured.

    If it really sucks , shouldn't be hard to remove the coating.

    Best grab a snorkle cos I'm going down this rat hole soon as deciding just how polished is polished.
    bmw r100s rear master dip pot - 1 (2).jpeg
    Last edited by gtgt_bangbang; 09-30-2022 at 02:46 AM.

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