GARAGE
MOTORCYCLES SITE | Ted's
House Motorcycle Tech |
Replacing
a Rear Brake Master Cylinder on a K75RT/ABS I. INTRO As a bit of background, about 5 weeks ago I pulled out for a ride and discovered no back brake, absolutely none! It was obvious from the jets of brake fluid escaping it that the rear master cylinder had failed and needed repair or replacement. I called BeemerBoneyard and managed to secure one of the few remaining Magura 12mm rear brake master cylinders for about half the cost of BMW OEM. Seems BMW had sued Magura to stop them from selling an aftermarket alternative - and once I compared the failed OEM unit with the Magura I knew why. They are exactly the same, down to both the Magura and BMW logos on each. Not too hard to figure out that Magura supplies to BMW, and was selling the same component through their own lines of distribution. II. PREP I called BeemerBoneyard and managed to secure one of the few remaining Magura 12mm rear brake master cylinders for about half the cost of BMW OEM. Seems BMW had sued Magura to stop them from selling an aftermarket alternative - and once I compared the failed OEM unit with the Magura I knew why. They are exactly the same, down to both the Magura and BMW logos on each. Not too hard to figure out that Magura supplies to BMW, and was selling the same component through their own lines of distribution. I also bought an extra bag of copper crush washers and a pint of brake fluid (which I forgot to bring and stopped at Autozone to get a liter bottle of Valvoline synth Dot 4 - good thing as we used a good half of it.). I brought my small Craftsman socket set, my BMW tool roll (with extra 11mm wrench for bleed nipples), and my trick metal 1-way valve (Amazon, $16 - avoid the plastic ones) brake bleeder hose. Those contained all the tools needed for this - but one. But I will get to that in a minute III. PROCEDURE DISASSEMBLY With the bike on the centerstand I removed the right bag and right side panel, and broke out the tool roll. Make sure to go slow and clean everything.
REPLACEMENT & ASSEMBLY
Voila! You did it! Now, bleed the brakes. I used my check valve bleeder and it worked just fine, first at the ABS can and then at the caliper, then repeat. Total time, about an hour. Difficulty level - simple. So the report is in on the failed MC. I removed the little allen to release the plunger and out spilled a mixture of brake fluid and sand. Only it wasn't sand, it was tiny bits of hardened brake fluid. The plunger itself seems to be in great shape - both rubber collars are undamaged, no tears or malformations. The bore looks great as well, there was a bit of discoloration and residue, that came right off with a bit of brake cleaner and a brass brush from a gun cleaning kit. Used the terry plunger from the same kit and it looks brand new. I have a feeling had I been on the road I definitely could have gotten home with a roadside take apart and clean, and probably could have simply resurrected it with a good cleaning and new rubber accordion boot. From the BeemerBoneyard page it looks like the boot is only $6. Looks like they still have some of the Magura MC's too at $135 each ($125 with your BMWMOA discount), a good deal less than the $250! BMW now wants. If I were to tackle this again I'd also get and replace the $8 reservoir hose. Mine didn't look so hot and it is sure easier to replace when you are already in there. Some Photos: Old Master Cylinder - notice the boot is so out of shape and malformed it easily falls off of the MC. An overview of the old, dissembled MC. Close up of the sandy consistency of the hardened brake fluid trapped inside the MC. Parts left over after the swap.
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