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BMW F650GS rear brake keep seizing? Try this…

Seems all my bikes are being blighted by brake issues. While the Ducati has reassuringly unpredictable electrical problems, the BMW has reassuringly German, old-fashioned mechanical problems.

So the symptoms were pretty simple. The rear brake jams on after a few miles of riding. Just happened out of the blue while commuting to work one day. Being a bit of a MacGyver type, I got my house key out and wiggled the brake pads back a bit. Wasn’t easy, especially as the brake was red hot from the friction. Took 10 minutes but it seemed to work eventually.

The rod that the pads slide along looked rusty so I assumed the winter road salt had done its worst. Same happened on the way home. So back at home, I stripped the brake caliper, regreased with copper grease and reassembled. Same happened the next day so that didn’t work! BTW. The rear caliper isn’t actually bolted to the swingarm. It sits on a free-floating ‘peg’ on the swingarm and is simply held in place by the brake disc running between the pads. So if you’re looking for the bolts to remove the rear caliper, you won’t find any. Just remove the rear wheel and the caliper falls off by itself.

I also adjusted the push-rod that connects to the Brembo master cylinder from the rear brake pedal. Same issue happened. I even took it off, bled all the brake fluid. Disassembled the Brembo cylinder, greased the internals checked the spring blah, blah… no joy. TIP. If you need to bleed the brake fluid and you have the ABS model, you need to rotate the rear wheel to push the fluid through. Took me ages before I realised.

As usual, online forums were full of confusing advice, as they were solving someone else’s problem, not mine. I have the ABS version, so lots of chat about the ABS sensor having issues.

Cutting to the chase, it WAS the Brembo master cylinder, but it needed a new one to solve the problem (or a rebuild kit I guess). It was as simple as the piston not quite returning to the resting position. The image above shows the ‘sticking’ one on the right and you can see the black piston is 5mm or so lower than the new one on the left. Therefore the brake fluid was being ‘pushed’ by the lever, but then being held there. The pads then stuck on, causing friction against the disc, causing heat, making the fluid expand, binding the pads even tighter. Explains why it took me 10 minutes of ‘fiddling’ that first morning before the brake worked again. It was the fluid cooling down and releasing the pads all by itself.

So I ordered a new master cylinder (they’re fitted to loads of bike models). Making sure I had the ‘in-line exit’ version (as the fluid came in from the side and out from the top… in-line with the body) and the slightly bigger piston version (13mm rather than 11mm) for more oomph. It’s part number 10.4776.60 so google “Brembo 10.4776.60” and you should find one. I got one from this page on IntoBikes.co.uk for £43. Putting the old and new one side-by-side, it was immediately obvious the piston (the white ceramic looking thing) wasn’t returning to the starting position against the circlip/washer like the new one did.

Bolted it on, new fluid (making sure to pump it through by rotating the rear wheel). Problem solved! Been riding it for 2 months, no sign of any problem.

There was some chat online about needing a special tool to bleed the ABS internals as air bubbles would cause it to malfunction, but I didn’t have any problems. Tried it out a few times and works fine.

So, as usual, bodged my way to another fix. Hope that helps if you were having similar problems!

6 comments

Leave a Reply to Sten G Wigert Cancel reply

  • Thank you my Griend.
    Picked up a 2008 F650GS a week ago. Came with 63,000km and brakes out of order. Front’s were easy. The rear’s stuck like yours. Blead from air and checked the piston return on the main. Problem solved.
    As you stated the problem is usualy simple.
    The cure: Get the air out and make sure the main cylinder returns and you will be riding again…

    Sten in Singapore

  • I have exactly the same problem by the looks of it. I was just wondering if the 13mm cylinder has the same thread to attach the lines towards the brake calipher?

  • One of my F650 rear master cyclinders lasted only a year before complete replacement, the internal white (piston?) bit seized in place on the upstroke. You do what you can to keep them clean, nurture them, bring them along and this is how they repay you, eh? Search for similar on this site, someone said they managed to strip and clean theirs.

    Cheap car service

  • Well, i just done the whole thing that dino said.
    The rear master cylinder is absolutely the same , just it works …
    Thank you for the advice my friend!!

  • Had exactly the same problem on my BMW G650 X Challenge which, it so happens, shares the same part. After a bit of toing and froing, cleaning and bleeding by me and an unsuccessful attempt at fixing it by my local bike shop I took your advice! 18 months later the replacement part is still going strong and I haven’t had an issue since – thanks!

  • Same happened to me with my fiancé’s BMW650 dakar! Rear wheel almost locked around a bend with loose gravel. I went the same route with caliper but with no success.
    Went to a BMW mechanic and he said he knows exactly what to do he has made some special tool to fix this for R480 (still a lot of money)

    So I decided to give it a try, drained the brake fluied and took te master cylinder off. There it was, axactly as the far top picture on the right, no matter what you do the piston stopped a few mm short. I washed it out and pushed it in and out a bit and it clearly helped but it was not enough. The teflon bush reduced in dia because of there tendency to ‘creep’ over time.

    I needed to open the teflon bush by just a fraction so played around with the back and front of my smaller sockets which had nicely rounded edges. When I found the one that just not would go in I help it on with a little tap of the hammer and Voilà, when I took the socket out with a plier and out popped the master cylinder piston!! I was so excited I completely forgot to take pictures!!

    Well it has been replaced, cleaned and primed and works absolutely perfect!! One happy fiancé! Only time will tell how long it will hold but at least we know what to look out for next time!

    Hope this helps someone else!