Rear Master Mystery

Discussion in 'Airheads' started by TINK, Feb 19, 2013.

  1. TINK

    TINK Been here awhile

    Joined:
    Nov 18, 2007
    Oddometer:
    772
    Location:
    So Cal
    After trying to bleed the rear brakes using every method known to man, I determined the master is the problem, RATS!:baldy

    Okay BMW brake experts, put on your thinking caps... :shog

    So here's the question; should the new piston, with the cup shaped seal, have holes in the front like the old piston?
    I'm thinking my problem is that with the new piston (no holes) I'm not getting brake fluid flow from the master reservoir to the brake caliper when I release the brake piston and it returns to the starting position?

    This is the old piston I took out, note the hole in the seal grove
    [​IMG]

    This hole goes through to the front of the piston, there are two of them.
    [​IMG]

    This is the old seal, note that it is not cup shaped but rather it is rounded like an o-ring...
    [​IMG]

    And this is the new piston with a cup shaped seal and no holes under the front seal.
    [​IMG]
    #1
  2. TINK

    TINK Been here awhile

    Joined:
    Nov 18, 2007
    Oddometer:
    772
    Location:
    So Cal
    Now that I have the master apart, again, I think the problem with the steel ring on the end seal that is press fit into the master was that it was on backwords.
    Looking very closely at the original one I noticed there is a tapper to one end, the end that slides into the master.
    The steel ring on the new end seal was backwards when I tried to press it in, and subsequently bent it slightly out of shape doing so.

    [​IMG]

    [​IMG]

    TINK
    #2
  3. Cogswell

    Cogswell Spudly Adventurer

    Joined:
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    Oddometer:
    6,000
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    Riding with my pal Richard Cranium
    What model and year of bike, there were some changes over the years.


    Mike
    #3
  4. TINK

    TINK Been here awhile

    Joined:
    Nov 18, 2007
    Oddometer:
    772
    Location:
    So Cal
    So, here's what I "think" is suppose to happen.

    [​IMG]

    I believe my problem with bleeding the system with the new cup shaped seal is that with air in the system there would not be enough hydraullic suction to fold the cup over and draw fresh fluied from the reservoir. The only logical (to me) way to fill the entire system with fluied is to open the bleeder screw then push fluied down from the reservoir side thus folding the cup down allowing fluied to pass the seal. What I tried, unsuccessfully, was to pressure feed fluied up the system from the caliper bleeder towards the master, this would only "lock" the cup in place this preventing fluied from passing.,

    Anyone? Anyone? Bueller?

    TINK
    #4
  5. TINK

    TINK Been here awhile

    Joined:
    Nov 18, 2007
    Oddometer:
    772
    Location:
    So Cal
    Sorry, 1979 R100RT
    #5
  6. Cogswell

    Cogswell Spudly Adventurer

    Joined:
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    Oddometer:
    6,000
    Location:
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    Looks to me like there are two options for the rear master cylinder, and two options for a rebuild kit depending upon date of manufacture.

    Maybe you have the incorrect one for your bike ? Check the part number against the fiche and let us know.


    Mike
    #6
  7. TINK

    TINK Been here awhile

    Joined:
    Nov 18, 2007
    Oddometer:
    772
    Location:
    So Cal
    I'm sure I've got the right part, the newer 1981-1984 piston is smaller by 1.58mm

    [​IMG]


    It appears the cup style seals are a superseded design over the old o-ring style seal.
    [​IMG]
    #7
  8. Cogswell

    Cogswell Spudly Adventurer

    Joined:
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    6,000
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    Riding with my pal Richard Cranium
    Yeah, I can see the pictures you've added now. I will agree with you that if that end seal is damaged fluid would be running out.

    FYI, the rear brake can be a beyotch to bleed unless you can get the caliper higher than the master cylinder.


    Mike
    #8
  9. DaveBall

    DaveBall Long timer

    Joined:
    Sep 30, 2009
    Oddometer:
    1,165
    Location:
    Vancouver Island
    Once you get the master fixed, put it on the bike and reverse bleed it. You don't need to do any strange handstands or incantations, and no black magic or sacrifices to the Brake Gods.
    Just get a good size syringe and some plastic tubing that will fit the syringe and the bleed nipple. Fill syringe with new brake fluid, open up the bleeder and slowly push fluid into the system till the master is full and no more air bubbles come out. This method will give you a good solid rear brake.
    #9
  10. TINK

    TINK Been here awhile

    Joined:
    Nov 18, 2007
    Oddometer:
    772
    Location:
    So Cal
    It took some doing, and was a GIANT PITA, but I've got rear brakes now. :clap

    After I got the end seal squared away by pressing the metal ring into the master bore with the tapered end inward, it was time to re-install the rear master and bleed the system one more time... and pray that it would work. :shog

    Using a syringe with a rubber taper tip that came with my Mityvac kit I forced brake fluid both up from the caliper and down from the master reservoir. I left the steel pipe fitting at the master loose for the initial bleed to allow air to escape from both ends of the system.

    The rear wheel was off to allow easier access to the caliper and to prevent accidentally getting brake fluid on the wheel or tire. Caliper was swung up to get the bleeder at the highest position.

    After I tightened the steel pipe fitting at the master I pumped the syringe up and down on the master reservoir port to force any remaining air out of the master. This produced quite a few bubbles, making me believe I was on the right track.

    Next I did a few cycles of the traditional pump the pedal and crack the bleeder screw.

    At this point I had a good solid feeling brake pedal, so I put the rear wheel back on and buttoned up the system.

    After pumping the pads back out to the brake rotor the pedal seemed to have a very long stroke, perhaps 3 inches, before the brakes would engage. :hmmmmm

    I decided to leave well enough alone for the day. Before retiring I blocked the brake pedal in the down position to allow any remaining air in the master to escape overnight.

    The next morning I was pleased to find that not only did I have a solid rear brake pedal but the pedal stroke was now about 1 inch. HALLELUIAH! :rilla
    Ya baby, I'm a BMW Rider now :bmwrider
    [​IMG]

    TINK
    #10