Wednesday, May 11, 2011

Leaking Front Master Cylinder

Yesterday evening, I disassembled the front brake master cylinder from my '83 BMW airhead to try and determine where it was leaking. There was some brake fluid where the piston shaft sticks out towards the brake lever as well as some coming out from between the plastic fluid reservoir and the master cylinder. After removing the internal snap-ring and pulling out the piston, a whole lot of "gunk" came out with the brake fluid. Even though I have been changing the fluid every year since I purchased the bike, there was still stuff floating around (the brake fluid itself was not discolored). I also decided to pull off the plastic fluid reservoir from the master cylinder to check on the condition of the O-ring and discovered the source of all of the "gunk". The reservoir should have come off easily after removing the single screw as there should have only been the friction from the O-ring holding it together. Apparently, the previous owner had used some sort of glue to seal the reservoir to the master cylinder instead of just relying on the O-ring. The glue has deteriorated over time and little flakes of it went into the brake fluid. There is still some traces of the yellow glue that looks like clear epoxy stuck to the sides of the large opening where the reservoir fits as well as some that has been washed into the cavity to the right by leaking brake fluid. This picture is after initial cleaning with alcohol.

The master cylinder bore was actually in pretty good shape with no trace of rust or pitting in the cylinder or the spring. The rubber cup is a bit worn and if you look really carefully, you can see very small cracks along it's edge. The rubber seal located midpoint on the piston has worn enough such that it no longer effectively seals out dirt and water from getting into the master cylinder from the brake lever. BMW does not sell a rebuild kit for this model and you have to purchase the entire assembly ($$$). I guess then I don't need to try and clean off the traces of glue. I've ordered a new cylinder and piston assembly, O-ring and gasket from the A&S BMW online store but have no idea how long it will take to get here. I've never ordered parts from them before so we'll see how it goes. When I had purchased the bike, there was a rebuild kit in the top box along with a receipt showing that it was purchased from the local dealer. Unfortunately, it was for the wrong model. When I looked on the online fiche to get part numbers, when I went to the model and year of my bike then went to the "brake" section, it showed the wrong master cylinder. The rebuild kit the dealer sold him was for this master cylinder. But when I went to the "steering" section, I was able to find and order the correct parts as they were listed as part of the right hand control. Weird...

I will probably finish cleaning things up and reassemble the master cylinder using the old parts and see how much it leaks while I wait for the new parts too arrive.

So again, I am seeing the value of the "spare bike". My wonderful wife signed up the the MSF BRC class for the first weekend in June. Over the weekend, we looked a bit at bikes and scooters but she wants to see how the class goes before making any sort of purchase. And we discovered that there were very few places in town that sold riding gear in any color except black or some fluorescent color with Yamaha all over it. My thinking was thinking that if she goes through the class, then she would be a safer passenger even if she never decides to get a bike or scooter. Dan Bateman even mentioned this in his most recent post (May 9, 2011) so I feel better about my request for her to take the class.


  1. Richard, always good to have a spare motorcycle...with my wrenching skills, sometimes two is not enough!

    I am working on getting my wife to go through the BRC as well, not only so she'll be safe on the scooter I am trying to talk her into for her 1 mile commute to work, but also if we're out riding together in the sidecar and I somehow get hurt to the point I can't ride, she can get us home....

    that's the theory anyways.

    Careful with rebuilding that old master cylinder...brake fluid eats paint as you probably know...


    Redleg's Rides

    Colorado Motorcycle Travel Examiner

  2. I can definitely recommend a spare bike. Going cold turkey... not good. Been there...

    Great to hear that your wife is taking classes. I find it so much better to spend some riding time with the better half. I hope she likes it!

  3. Glad you located the source of the spewage. After 28 years, I guess its fair for the rubber to be cracking and breaking down. Here is hoping the replacement parts come expediently. I've never been good at waiting for parts.

    Since gas hit $4 a gallon here, scooter sales have gone thru the ceiling. I don't know why they didn't catch on earlier.

    My wife is lobbying for a spare bike, namely a Harley Trike. We shall see.


  4. Great news that you found the problem. Sorry that you're back to the cage for a bit. It shouldn't take too long for the parts to arrive from Calif. Another good source for parts you might already know about is Beemer Boneyard. He's very helpful on the phone too.

    That's great that your wife is ready to ride. It'll be fun to read about your adventures together.

  5. Charlie6:
    So far, no issues with paint. The fairing doesn't touch the tank so the fluid that has dripped onto the fairing glove box never got near paint.

    Yep, the scooter is starting to look better and better. I did see a F650GS thumper in Wrangell with only 2K miles on Craigslist but he had already had someone coming to look at it.

    My wife has mentioned a Harley trike several times as well but I keep responding with sidecar...

    After a lot of thought, I decided to order a new master cylinder instead of getting a used part. After all, anything I get will be just as old. I've never ordered anything from the Beemer Boneyard before. Thank you, I should look into it...