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Thread: Rear brake fluid reservoir cap question

  1. #1
    Registered User ezwicky's Avatar
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    Rear brake fluid reservoir cap question

    I asked this on the Airheads list but no reply yet, so I want to ask here as well:

    On my '80 R100RS with Brembo rear disc brake, the lid of the fluid reservoir has terminals for low fluid warning, like the under-tank one for the front brakes, but the doughnut-shaped float does not move up and down like the front one does.

    It is completely rigid, no play up or down. Fixed to a post that seems molded into the underside of the lid.

    It's all plastic, nothing I can see to rust or freeze up, and the rest of the reservoir looks fine, the fluid was clear, no gunk in it.

    Replacement reservoirs have a flat lid with no provision for low-fluid light, so even if mine is "bad", the replacement ones don't function either, so I'll stick with this one I guess.

    But how does the fluid level open or close the switch? I took the terminals off, cleaned them up and tried to get a peek inside but the orifice that the screws go in is too small to see anything.

    Thanks in advance,

    Eric
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  2. #2
    Liaison 20774's Avatar
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    My guess is something has been substituted along the way. I would check Brook Reams website for his overhaul of several RS bikes. I'm sure he's covered this.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User ezwicky's Avatar
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    Quote Originally Posted by 20774 View Post
    I would check Brook Reams website for his overhaul of several RS bikes. I'm sure he's covered this.
    Haha yeah that's the first place I always go before asking questions Brook deserves a medal for his amazing documentation, but in this case there's no mention of the reservoir cap / fluid-level switch function.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  4. #4
    Liaison 20774's Avatar
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    We had a discussion about a '77 R100RS rear reservoir over on the Vintage BMW Owners forum:

    https://vintagebmw.org/forum/viewtopic.php?f=8&t=16522
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
    Registered User ezwicky's Avatar
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    In that discussion, Steve Rankin says "The float drops with the drop in fluid, closing the circuit to turn on the bulb in the dash."

    Which makes total sense. BUT.... mine does not drop. The entire thing seems rigid. The "float" is attached to a seemingly molded-in post which does not move up or down.

    AND... the aftermarket reservoirs have no low-fluid switch, so I guess you just cap off the existing wires if you buy one of those?

    Something is weird.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  6. #6
    Liaison 20774's Avatar
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    So how certain are you that your current reservoir/cap are stock and original? If you have electrical terminals on top of the cap, that speaks to the fact that there needs to be some kind of sensor involved.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #7
    Registered User ezwicky's Avatar
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    Yeah that's what is bugging me. Terminals are there, and in the cap there is a float-looking thing, but it does not "float". I can't be certain about it being original.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  8. #8
    Gerard jagarra's Avatar
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    It could be that a portion of the float mechanism is missing. If there is a hole in the center of the bottom of the cap and you have a meter, put the meter across the terminals and insert something into that hole to see if you can close a contact.
    1994 R1100RS-(5/93)-,1974 R90/6 built 9/73,--1964 Triumph T100--1986 Honda XL600R

  9. #9
    Registered User ezwicky's Avatar
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    Mystery Solved

    OK I took apart further the cap and there is indeed a very small metal post that is bound up. I knew there had to be something like that, but until I took the cap apart completely I couldn't see it. Hopefully I can free that post, which attaches on one end to the float, and the other end closes the switch contacts.

    But it's still interesting that aftermarket caps have no provision for a low-fluid switch. I've looked at EME, Boxer2Valve, Motorworks and Motobins. And of course Max's fiche.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  10. #10
    Liaison 20774's Avatar
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    If you can, post some pictures to give another owner in the future an idea of what to look for.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  11. #11
    Registered User ezwicky's Avatar
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    pics

    Quote Originally Posted by 20774 View Post
    If you can, post some pictures to give another owner in the future an idea of what to look for.
    I can't add pictures. I get all the expected dialog boxes, but for one thing, I can't select multiple images. But even if I select one and click "upload" button, nothing happens. I'm using Chrome.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  12. #12
    Liaison 20774's Avatar
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    Quote Originally Posted by ezwicky View Post
    I can't add pictures. I get all the expected dialog boxes, but for one thing, I can't select multiple images. But even if I select one and click "upload" button, nothing happens. I'm using Chrome.
    Are you using the little picture icon above the message screen:

    PictureButton.jpg

    You Choose File, navigate to your file on the computer, then upload the file. You'll have to do things individually but it's really fairly straightforward. I just used my Chrome browser (I normally used Edge) and had no problems...I'm working in Windows. Note that the image needs to be reduced in size, say 1Mb is size up to no bigger than 2Mb. Bigger is not necessarily better in these situations.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  13. #13
    Registered User ezwicky's Avatar
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    Quote Originally Posted by 20774 View Post
    Are you using the little picture icon above the message screen:

    PictureButton.jpg

    You Choose File, navigate to your file on the computer, then upload the file. You'll have to do things individually but it's really fairly straightforward.
    Yup. I am really familiar with uploading files to vrious forums and websites. Somehting is just hosed , maybe on my end, who knows.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  14. #14
    Registered User ezwicky's Avatar
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    I wonder if it's a file-size thing. I'll see...

    No that's not it, the largest one is 135kb.
    BMWMOA #182796

    '76 R90/6
    '80 R100RS restoration project

  15. #15
    Liaison 20774's Avatar
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    Should work with a file that small. I assume they are JPGs. You hang out of various forums, so this shouldn't be that difficult. It's really easy, though, so I'm scratching me head.

    You are trying to upload from your computer? Not sure how that would work on a smartphone... I just did it from my phone...no problemo...
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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