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Thread: 83 R80RT 15 mm master cylinder rebuild kit

  1. #1

    83 R80RT 15 mm master cylinder rebuild kit

    Hi all, I recently was riding my uncles 83 R80RT when I noticed some drips coming from the front brake master cylinder. I thought no problem I will just grab a rebuild kit and put it in for him. I got on beemer boneyard and was surprised to find I could get several different sizes but no 15mm. Through some searching I find that it has a dual front brake conversion on it. The only way I find any mention anywhere of a 15mm master cylinder is through the parts fiche on max bmw. The only way they sell it is an entirely new master cylinder for $183.00. Has anyone ran into this problem before and been able to find a rebuild kit for the 15mm anywhere?

  2. #2
    Liaison 20774's Avatar
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    Bench Mark Works - http://www.benchmarkworks.com/ - lists a 15mm kit on their online store.

    On the online parts sites, it seems that the '83 R80RT came stock with the 15mm handlebar master cylinder. Does your bike have twin disks?
    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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    Registered User GTRider's Avatar
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    My experience has been that when you reach the point where the MC is leaking you are money ahead to just buy the $183 master cylinder and be done with it, even tho a rebuild kit is half the price. Chances are very good that when you pull the piston youíll see corrosion in the bore, and adding the cost of sleeving the MC raises the cost above that of a new MC. Plus, with a new MC you get a new reservoir with unclouded plastic and new o-ring between reservoir and piston. That o-ring is often the source of continued leakage after installing a new piston kit. And finally, itís half the labor to replace instead of rebuild. Donít forget to order new crush washers for any banjo fittings. I think Bobís BMW has 15% discount until the end of the month, if that helps with the cost decision.

    Best,
    DG
    DGerber
    1983 R80ST ó 1984 R80 G/S PD ó 1993 R100GS ó 2004 K1200GT w/Hannigan S/C ó 2010 K1300GT
    BMWMOA#52184, AMA#271542, IBA#138

  4. #4
    Quote Originally Posted by GTRider View Post
    My experience has been that when you reach the point where the MC is leaking you are money ahead to just buy the $183 master cylinder and be done with it, even tho a rebuild kit is half the price. Chances are very good that when you pull the piston youíll see corrosion in the bore, and adding the cost of sleeving the MC raises the cost above that of a new MC. Plus, with a new MC you get a new reservoir with unclouded plastic and new o-ring between reservoir and piston. That o-ring is often the source of continued leakage after installing a new piston kit. And finally, itís half the labor to replace instead of rebuild. Donít forget to order new crush washers for any banjo fittings. I think Bobís BMW has 15% discount until the end of the month, if that helps with the cost decision.
    +1

  5. #5
    Registered User kioolt's Avatar
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    I've had the opposite experience about a leaking front master cylinder. It seemed that when I did a fluid flush I would get a leak at the master cylinder. After I replaced the piston the leak stopped. No leak until the next fluid flush. Replace the piston again. Leak fixed. I then would only pull the lever part way to the handlebar when doing a fluid flush and no leaks. That's what I did for several years afterward without problems. Your mileage may vary.
    2004 R1150RT 186,800 miles , 1991 K100LT 128,700 miles, 1982 R100RT 106,900 miles
    Total 422,400 BMW miles
    AMA,BMWRA,BMWMOA
    The cheapest thing on a BMW is the nut that connects the handlebars to the seat.

  6. #6
    Liaison 20774's Avatar
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    Quote Originally Posted by kioolt View Post
    I then would only pull the lever part way to the handlebar when doing a fluid flush and no leaks.
    I'm just finishing up an overhaul on my /7's under tank master cylinder. But the above is something to consider when doing routine fluid replacement. Moving the hand lever well outside the normal range can invite situations where the internal seals are torn on deposits, pits, or rust that might have built up in the bore...doing a full pull on the hand lever can cut the seals. Best to put a small piece of wood on the handlebar to limit the travel on the lever.

    I found some small rust spots in the bore of my master cylinder. I followed Brook's excellent advice and pictures for his R100RS rebuild. I used some 600 grit sandpaper wrapped around a socket with an extension and used it to nicely hone the bore. Wish me luck!!
    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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