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I just did my ~24,000 service on my 2008 R1200GS Adventure with just under 60,000 miles on it, a task I've done several times before on this and previous 1200GS bikes.

As part of that service I did a brake fluid flush, a task I've done before (also have done brake pad replacement before) on this bike, no previous problems. As usual, I wiggled the rear caliper a little before removal from rotor to slightly spread pads to ease re-installation afterwards.

This time, however, while front brakes bled fine and are working AOK, rear brakes are non-functional. Have pumped the pedal maybe 100 times in a row and can't get anything like pressure in the rear brake system with bleed valve closed. With bleed valve open have no fluid coming out of valve, and fluid level in rear reservoir remains unchanged. Pedal pumps up and down and apparently does nothing.

Thinking the rear brake master cylinder is bad, but before I order a $217 part I thought I'd pass the problem by the collective wisdom found here. :)

Could the problem perhaps be air trapped in the rear caliper? I didn't fully depress the pistons when I took the caliper off the rotor, just wiggled the pads a bit loose to ease re-installation afterwards. Still, even if air is trapped in rear caliper, wouldn't brake fluid still come out of the bleed valve, and leave the reservoir, when bleeding (but the trapped air in caliper would leave pedal soft)?

If it does come to replacing the rear brake master cylinder, anything special I should be aware of? Do I just unbolt the old one, bolt in a new one, and run a bunch of DOT4 through the system, normal bleeding style? (I do have the GS-911 for doing the ABS bleed function.)

I should also mention that I've almost never touched the rear brake in nearly 60,000 miles, using it only very rarely, but I did use it a few times in the last month and it was working OK at that point. Also, I don't see any fluid leaking from anywhere, not rear caliper, not master cylinder, not reservoir.

Thanks!
 

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A few ideas fwiw

Have you checked reservoir hose? If this has deteriorated then it might not be feeding fluid to m/c. Remove reservoir and hose and check liquid flows through. Rubber does die over time

M/c not rebuildable from what I recall, but you can remove and clean it up with some silicone spray. you should be able to determine if it works by attaching a good reservoir hose and pumping fluid out of it

You can use a vacuum type bleeder to pull fluid through.

Seems unlikely to be blockage in caliper, but you can test this by pushing fluid through bleed nipple with syringe and checking it comes out clean from banjo hole

If you isolate and check each component you should be able to narrow down the location of the issue

one other point to note is that if the lines have no fluid, it can be a pita bleeding them. If you pump pedal multiple times and bleed quickly fluid will eventually start moving
 

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What was the problem with your rear brakes? I have a similar issue with mine on a 06' GS. The rear brakes are non functional.
 

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What was the problem with your rear brakes? I have a similar issue with mine on a 06' GS. The rear brakes are non functional.
Brakes on 2004 - 2006 are different from 2007+. Have you tried the rear brake the engine running. Is there an improvement in pedal feel. Most of the components are the same save for the servo assist part that is know to go tits up on 2004 - 2006 models. The servo/ABS brakes the bike need to be powered on or you only have residual braking (about 90% less than normal). Also is the battery in good health? This can also cause problems on rare occasion.

If the servo/ABS unit is given up the ghost you have several choices.

1. Try properly flushing/service the ABS/servo pump. There is a process for this that requires more than traditional brake bleeding.
2. Buy a new servo ABS unit from BMW for $3,000. This is for the part only no labor for install or programming figured in.
3. Bypass the servo/ABS pump and have great non-ABS brakes.
4. Ride with eh rear brakes failed. Nort suggested but you would be the 1st person who has done it.
5. Sell the bike with full disclosure of the failed brakes normally this means discounting the price of the bike substantially as those in the know will know the cost of repair.

Is your ABS light on and/or blinking rapidly.

Good luck!
 
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