The Brake Repair From Hell

Back on the twenty-fifth of May the Magic Carpet developed a leaking rear brake line that was later tracked down to being the hose from the rear master cylinder to the metal brake line going to the ABS unit.  On the twelfth of June we went to the dealer fully expecting to have to order the offending part.  Well we got lucky and they actually had the part.  I got the repair under way on the thirteenth of June.  Yes on Friday the thirteenth!  I removed the body work that hadn’t already been removed to locate the leak, removed the old brake line and then managed to get the new line in.  The hard part was getting the new line in!  There was absolutely NO pressure on the rear brake pedal.  But by opening the system, I knew I had introduced air into it and it needed to be bled.  I was beat and done for the day.  On the fifteenth I tried bleeding the system no avail (I later found I wasn’t patient enough).  I was flustered and asked for help on bmwlt.com for what to do.  An experienced “wrencher” on the K1200LT suggesting back flushing the line using the #3 bleeder with a syringe and a piece of tubing to fill it.  I already had those from my last brake repair!  It still wouldn’t bleed.  I took a few days off from the project because it was frustrating me.  Then another “wrencher” suggested that at this point since all I wanted to do was get the air out, was to connect the tubing to each of the 3 rear bleeders from the ABS unit back into the brake reservoir so that the air would come out and I would just recycle the brake fluid.  I would worry about a more thorough bleed and flush after this.  I now had pressure on the brake pedal!  It took a lot of time to get that air out too.  That was on Saturday the 21st.  We had decided that if I didn’t get this done by Monday we were sending the bike to the dealer to let them figure it out.

Now it’s time to explain a BMW servo assisted (power brakes like a car) integral ABS (adding “braking system” to ABS would be redundant, like VIN number) unit in layman’s terms as easily as possible.  The integral part means that pressing the rear brake pedal will not only give you rear brake but will add front braking with pushing harder.  When pulling the front brake lever you will also get rear braking as you pull harder.

  • There are a total of 4 circuits.
  • The front wheel circuit goes from the ABS unit to the two front calipers.
  • The front brake lever is for the front control circuit from the lever to the ABS unit.
  • The foot pedal is the rear control circuit going from pedal to the ABS unit.
  • The rear wheel circuit goes from the ABS unit to the rear caliper.
  • There are 6 bleed valves on the ABS unit (3 for the front and three for the back), one bleed valve on each of the front calipers and two bleed valves on the rear caliper.
  • The reservoir for the rear brakes feeds both the ABS unit and the rear caliper
  • The reservoir for the front brakes feeds only the front calipers while the master cylinder on the handle bar supplies only the ABS unit.
  • All lines meet at the ABS unit in the middle.

Confused yet?  A diagram really helps though.  For the home mechanic to do this it really is a two person job.

Yesterday, on Sunday, we bled and then flushed not only the rear system but the front system as well.  I figured since I was in there I’d do them all.  It’s also much easier to do the control circuits on the ABS module with the battery removed from the motorcycle.  To do the wheel circuits you need the battery installed because the ignition has to be on (but bike not running) because you need the servos the flush from the bleeders on the calipers.  As expected it took some time to do the rear control circuit.  Donna was my “beautiful assistant” manipulating the brake pedal and brake lever for me.  The front control circuit went quick and the fluid didn’t even look that dark but now the whole system has new DOT4 brake fluid.  While the Magic Carpet was still naked, I took her for a short test ride and got no brake warning lights or leaks.  I put the brakes through a workout, even intentionally activating the ABS a few times.  I got her home and then put all her clothes (body work) back on.  I did lose 3 torx screws for the body work in all this though, but that’s no big deal.

Now it’s time to start riding again!

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