Friday, May 13, 2011

Front Master Cylinder Repair

I lost a couple of posts due to the still unexplained Blogger outage. Here is combination of the two lost posts.

Update: The lost posts reappeared automagically but the comments are missing, Thank you to all who comments were lost...

On Tuesday evening, I took apart the leaking front master cylinder from my '83 BMW airhead to see where it was leaking from and discovered that the previous owner had used clear epoxy to glue the fluid reservoir to the master cylinder. The brake fluid had pretty much dissolved the epoxy making a real mess of everything with a lot of "gunk" floating around in the brake fluid. Since BMW does not seem to sell a rebuild kit for this particular model of master cylinder, I went ahead and ordered from A&S BMW a replacement. I thought about going to the local dealer but the previous owner had purchased a rebuild kit and they sold him the kit for the under tank master cylinder. That is one listed on the microfiche for the 81-84 R100RT but if you look under "Steering", they show the correct part as part of the right handgrip. Weird.

Here is the piston and, as you can see, there is no rust on the spring but the rubber parts are pretty worn out evidenced by the small cracks at the sealing edge of the cup. As Mike pointed out in one of the lost comments, after 28 years a little wear should be expected. The rubber seal near the middle of the piston is supposed to keep out water and dirt and it fits pretty loose within the cylinder. After finding out that A&S does not have the part in stock and that it may be weeks before getting it in, I decided to temporarily repair the master cylinder and get the bike back on the road.

On Wednesday evening, I used a very sharp knife followed by a very fine abrasive pad to clean the glue out of the opening for the fluid reservoir until it was smooth and the old O-ring (yes, I know that it is supposed to be replaced every time the reservoir is removed) should seal just fine. I also ran the pad up and down the bore and got a really fine crosshatch pattern. Basically just cleaned things up really well. There was one place where you could tell that the rubber cup on the piston rests and this procedure removed that little ridge.  Cleaned up the master cylinder and piston with alcohol, lubricated the piston and O-ring with clean brake fluid and reassembled everything. Installed the master cylinder on the bike and poured in some brake fluid to check for leaks. None so far! Bled the air out of the system and the lever felt solid again. I used the lever to bleed the brakes as it works better than the hand vacuum pump so this meant that the piston got used a lot. At this point there was a tiny drop of fluid which looks like it may have came out of the piston end. Filled up the reservoir to the "Max" line. cleaned up everything, and put some clean paper towels under the master cylinder. This morning, no leaks so I went ahead and rode into work. I put down some rags on the fairing glove box to catch any leaks and now I'll just have to wait and see. At least it looks like I may be able to still get out and ride until the parts arrive. Gotta love the simplicity of these old bikes. I don't think I would be so willing to tear into anything current.

Friday morning - The good news is so far, essentially no leaks. There was a small drop yesterday morning but since then, nothing. No problem waiting for the parts to arrive as the bike is still on the road.


  1. Now that they are getting blogger back on line all the comments have gone into the void. well bugger them. I still think if you had a current bike it wouldn't need fixing so you wouldn't have to fear trying to fix it. Something like that. I forget, but that was the gist of my original comment.

  2. Conchscooter:
    You are assuming that I don't enjoy fixing things. I think that is one of the requirements to owning an old BMW, you have to be willing and able to fix things. I despise having to take our Toyota into the dealer for all the small stuff which they charge a fortune to fix.

  3. Posts here, posts missing...It's been a crazy blogger week! I think things are coming back in bits and pieces.

    You have old bike syndrome. We suffered many years from old car syndrome. You can tell by the type of spare parts you keep around in your vehicle or garage.

    It was Hondas that finally got us to using the dealer shop. They find some confound way to locate the oil filter so you practically have to remove the engine to get to it.

    Remember the when you could tell what was wrong with your car just by listening to it. Valves that need adjusting, a u-joint going out. You had fair warning with some things. Now the car just quits running and you have to tow it to the dealer to be diagnosed only to find out a stupid sensor needs replacing. I want another 67 Camaro - low-tech and wicked sexy!

  4. Dear Richard M.:

    I have a deep respect for guys who delve into the mysterious world of ancient motorcycle repair. I cannot help but note that very other page of a BMW owners manual is rife with informative tidbits like:
    "Failure to properly adjust the XYZ may result in injury, or a horrible death complete with unendurable pain." That keeps me taking things back to the shop. But I really like and respect my mechanic — the famous Tom Cutter.

    Fondest regards,
    Jack • reep • Toad
    Twisted Roads

  5. Jack:

    And here I have just been diving in. I guess I should start reading the manual and at least acknowledge the dire warnings.

    I've heard wonderful things about Tom Cutter's skill with BMWs.

  6. This comment has been removed by a blog administrator.

  7. how do u remove that piston assembly from the master cylinder? i find my plunger is seized solid