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Thread: 74 R90/6 master cylinder question

  1. #1
    Registered User dmftoy1's Avatar
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    74 R90/6 master cylinder question

    Prior owner was honest man and I've got a receipt for a master cylinder rebuild kit so I've no doubt that he rebuilt it in the last six months.

    Today while tearing down in preparation for repaint/restore I notice so brake fluid on the frame near the rear of master cylinder. (not completely sure its not seeping from reservoir)

    Anyway, make the decision to take off the master cylinder, caliper, etc. so I can set it aside to eval and when I disconnect the rubber hose from the brake hard line the expected brake fluid comes out but so does all of the fluid in the MC reservoir. Am I nuts in thinking that I should have only had the fluid in the lines drain and not the entire reservoir? I've done tons of car/truck and motorcycle brake jobs over the years but I've never seen this happen. The brake cable was disconnected so there was no "tension" on the master cyl.

    Trying to figure out what my next steps are. From the frame corrosion its obvious that leaking brake fluid is a long standing problem, and I'm hesitant to rebuild it AGAIN in the hope that it would fix it.

    Any advice gratefully accepted.

    Have a good one,
    Dave

  2. #2
    rabid reader dbrick's Avatar
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    A small hole in the wall of the master cylinder just beyond the piston's "rest" position permits brake fluid from the reservoir to enter the working part of the hydraulic system. With the master cylinder piston in "rest" position - not moved down its bore - the small hole is uncovered; when the brake is applied and the piston moves down the bore, the first thing that happens is the piston seal passes the hole, thus disconnecting the reservoir from the working (higher pressure) part of the system.

    If the brake system is opened downstream (I think that's what you mean by "...disconnect the rubber hose from the brake hard line...") with the master cylinder piston in its "rest" position, brake fluid will flow from the reservoir into the master cylinder's working chamber, then down the brake line and right out the opened fitting. It's not surprising at all that the reservoir drained.

    The piston and seal may be OK, with the leak coming from where the reservoir fastens to the top of the master cylinder; I don't know how that joint is made. It's also possible that the rebuild was defective, or became defective sometime after the work was done, and that brake fluid was passing around the seal and piston, and then finding its way out of the master cylinder body and onto the fame.
    David Brick
    Santa Cruz CA
    2007 R1200R

  3. #3
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    Here is a link to a thread about master cylinder issues:
    http://www.bmwmoa.org/forum/showthre...ferrerid=51262

    And here is a link to Mike V's under tank rebuild pics. Shown is the o-ring between the reservoir and cast iron body. I expect that is the leak point - it seems to be on mine.

    http://s428.photobucket.com/albums/q...0MC%20Rebuild/

    I installed a stainless brake line, which may have generated some issues. However, I think (and will eventually know for certain) that the reservoir o-ring has a slight leak. I rebuilt the master cylinder along with having Apple Hydraulics install a brass sleeve. The brakes are good, but there is a slight brake fluid leak. If it were not for the ruined paint on the master cylinder, it would be difficult to determine there were any leaks at all.

    Edit: your observation about the brake fluid coming from the master cylinder is typical. In fact, that is the way I change the fluid in all my vehicles. The bleed screw is opened and fluid added to the MC. The technique works on ABS systems also. Air in the lines (after replacing a line) can be removed in this way - although sometimes pressurizing the MC is needed.
    Last edited by Stan_R80/7; 09-09-2012 at 03:05 AM. Reason: add comment
    Stan

    AH# 13238

  4. #4
    Registered User dmftoy1's Avatar
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    Thanks guys! Feel much better now knowing it'll "drain" while in rest position. I'll check that o-ring and see if I can't spot anything obvious. (brakes were working fine so I "think" the rebuild was ok)

    Very much appreciated!

  5. #5
    Monza Blue 1974 R90/6
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    All the above is good info. Hopefully, you will luck out and discover that the problem is the o-ring between the plastic reservoir and the metal body.

    However, it is possible that there is some pitting or corrosion in the sleeve and if it is bad enough, even the rebuild wouldn't have solved the problem because the fluid can get past the seal because of the pitting.

    I think one way you can confirm if this is where your fluid is coming from is to put a small piece of paper towel inside the rubber "boot" that covers the cable end and the actutating arm. After riding for a while, next time you have the tank off if you pull the boot and find the towel is damp, you'll know your leak is from the cylinder.

    From your other posts, it sounds like you have the bike totally apart, so it is probably a money question. If money was no object, get the cylinder resleeved (described above) while you have everything apart and get another rebuild kit. Although, I don't think the reservoir o-ring is part of the rebuild kit, so you'll have to get that separately. If you are balancing expenditures, just replace the o-ring and put the towel in the boot, ride it for awhile and see. If it is the cylinder leaking, that can be next years project.

    Barron

  6. #6
    James.A
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    There is a shop in California called "Sierra Specialties" that will fit a brass sleeve and hone it to size. I had it done and am quite pleased. brakecylinder.com if I remember correctly.

  7. #7
    Registered User dmftoy1's Avatar
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    Quote Originally Posted by Barron_Williams View Post
    All the above is good info. Hopefully, you will luck out and discover that the problem is the o-ring between the plastic reservoir and the metal body.

    However, it is possible that there is some pitting or corrosion in the sleeve and if it is bad enough, even the rebuild wouldn't have solved the problem because the fluid can get past the seal because of the pitting.

    I think one way you can confirm if this is where your fluid is coming from is to put a small piece of paper towel inside the rubber "boot" that covers the cable end and the actutating arm. After riding for a while, next time you have the tank off if you pull the boot and find the towel is damp, you'll know your leak is from the cylinder.

    From your other posts, it sounds like you have the bike totally apart, so it is probably a money question. If money was no object, get the cylinder resleeved (described above) while you have everything apart and get another rebuild kit. Although, I don't think the reservoir o-ring is part of the rebuild kit, so you'll have to get that separately. If you are balancing expenditures, just replace the o-ring and put the towel in the boot, ride it for awhile and see. If it is the cylinder leaking, that can be next years project.

    Barron
    I've decided with this project I'm never going to break even so money isn't a huge issue (although I don't want to waste any)

    My big worry is powder coating the frame and having it eaten away by a leaking master. May have just talked myself into re-sleeve/rebuild.

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