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-   -   Weak rear brake (http://www.horizonsunlimited.com/hubb/bmw-tech/weak-rear-brake-66126)

goo 6 Sep 2012 14:58

Weak rear brake
I think there is something basic I am missing here....

My rear brake seems weak. The pads and fluid levels are fine but I have to step on the pedal all the way down to get any real effect and even then it seems half-hearted. I don't know how to tell if the brakes need bleeding but in any case, my Haynes manual says I would need a dealer to do this for me as its an ABS system.

I'm sure this is a very basic issue with a simple adjustment to fix it but it eludes my novice brain!

Thing is, last time I took the bike to a dealer he somehow tightened up my front brakes so they were nice and responsive but I dont understand how. The fronts, a month later, are a bit sloppier (I put new pads in a week ago) and I'd like to tighten them up a bit too.

There is of course an adjuster that brings the brake lever closer to the handlebar but thats not it. Same goes I assume for the adjusting nut on the rear brake pedal. I guess these just adjust position and not responsiveness

Thanks for any suggestions

John Downs 6 Sep 2012 15:37


Common reasons for your rear brake symptoms:

old tired brake fluid or excess air in rear brake line.
solution: flush and bleed rear brake line.

oil or brake fluid on rear brake pads.
solution: replace rear brake pads.

weak seals in rear brake master cylinder or caliper.
solution: rebuild or replace.

As for your front brake feeling a bit mushier after you replaced the front pads recently, I imagine it is a simple matter of bleeding the brake lines. This is probably what the dealer did to tighten up your front brakes when they serviced the bike previously. It is easy to let a little air in the line if you crack the bleed screw to release caliper pressure to allow room for the new pads.

Others may have ideas.

Best luck,
John Downs

BlackDogZulu 6 Sep 2012 16:46

Sounds a lot like air in the brake lines to me too. The Haynes manual for my 650GS says the same as above. Apparently, bleeding an ABS system requires 'special equipment' that only BMW dealers have. Nuts to that. My old Honda ST1300 had ABS and linked brakes, and the pipework for that was horrendously complicated. Nevertheless, there was a recognised bleeding procedure which was lengthy and complex, but perfectly do-able by a reasonable DIY mechanic. I think this is BMW being a bit precious.

I'm guessing the 'special equipment' is a vacuum bleeder, something like a Mityvac - readily available, if a little costly. Anyone know? (Would be worth bookmarking for when I need to do this to the GS.) Might be worth trying a conventional bleeding method on the rear brake line before calling the cops.

goo 6 Sep 2012 17:05

Thanks for both those responses. Being as I am in a smallish town in Bolivia and never having attempted to bleed brakes before, any suggestions as to what I might be able to do? i.e. what is the conventional method? Or do I just need to wait until I get to Chile in a week or so?

John Downs 6 Sep 2012 18:34

I checked over on advrider and found this post from a fellow with a similar problem with his F650ABS brakes:

Bleeding rear brakes on BMW 650 with ABS? - ADVrider

The trick turned out to be removing the rear caliper and pads, slowly pressing the brake down till the piston nearly fell out, then sitting the bike fully upright (Dakar's lean way over on the sidestand) and pushing the piston all the way back in. Sort of a "reverse" bleed. Problem solved.

This youtube video might help with removing the rear F650 caliper and pads:

Technical Demo on BMW F650 - Rear Brake Pads - YouTube

Seems like something you could try in the field in Bolivia.

Best luck,
John Downs

goo 6 Sep 2012 19:09

Thanks John!

Please excuse my ignorance here, but do I just have the piston come nearly all the way out and back or do I need to open a bleed valve as I do this too?

John Downs 6 Sep 2012 19:46

The former. With the rear brake pads removed and one thumb on the rear piston puck, gently push down on the rear brake pedal forcing the rear brake piston out partway (not too far, maybe 1/2")before forcing it back in to push the brake fluid (and hopefully any air bubbles) back through the lines and into the rear brake master cylinder. You may have to lever the rear brake piston back in carefully with a screwdriver or somesuch as it requires a fair amount of pressing to move back in.

Not terribly difficult to do and I would give it a try if in your circumstances out on the altiplano with crap rear brakes and the nearest BMW dealer far away.

I have done this on other bikes but don't own an F650ABS. Just relaying helpful information in case it is of use. I feel your pain. No fun being in BFE with crap brakes.

Kindest regards,
John Downs

goo 6 Sep 2012 21:02

I've now done this - piston in and out 3 times but unfortunately it hasn't helped. Maybe the air is in a different part of the system. Guess I will just have to take it easy until I can get a pro onto it...

Nevertheless, as always, your help is much appreciated!

goo 6 Sep 2012 21:31

Talking of basics... I have realised in the process that I have no idea of how the wear indicators work on my rear pads :doh: I haven't worried too much about this until now as I can clearly see at least 3mm of friction material on both sides and because I barely use the rear (apart from offroad)

Anyway, there are no grooves as per the front pads and no chamfered edge as it says in my Haynes manual. What other kind of wear indicators are there? The dealer who serviced the bike about 2000km ago said they had about 4000km to go. Did he just measure their thickness?

I should just go back to my Raleigh Arena I had when I was 8. Knew where I was with that..:rolleyes2:

markharf 6 Sep 2012 22:58

I don't trust 'wear indicators' in general on brakes--too many opportunities for uneven wear or other faults (like the stones which sometimes get embedded in my mountain bike disk brakes, or the chunk of metal which once did the same in my KLR brakes). I just visually inspect thickness once in a while--when I change a tire, when it occurs to me, when I hear or feel something odd. It's not rocket science: they wear out, and when they get thin you replace them.

There are a hundred reasons for weak brakes. To begin with, I can't see any reason why changing pads should introduce air into the system. Maybe that's an ABS or BMW thing (somehow), but I've changed pads by just unclipping, pushing pistons back in their bores, clipping in new pads and bolting calipers back into place. As long as the whole mechanism is free to float, it self-adjusts quickly.

I have had the floating portion of a caliper get bent when tried to ride off with disk lock in place (insert facepalm indicator here). That prevented even engagement of pads, which in turn wore out one pad of the pair unevenly and very rapidly. Another reason the same thing might happen is if caliper bores get pitted or in some other manner stop letting one or more pistons to operate freely. One signal is uneven wear--another good reason to check visually. Another is weak braking, since you're not getting full, even engagement of both pads.

Another possibility is glazing due to overheating, possibly because pistons are sticking. Yet another is contamination by, for example, leaking fork seals, blowing engine oil, or sloppy chain lubing. I'd sure check all these things before bleeding.

Caveat: I'm no mechanic, and I don't know anything about fancy ABS-servo-assisted-electronic-controlled systems of any sort. Stuff I suggest might have absolutely no applicability to your bike. However, I hope it's helpful.


BlackDogZulu 9 Sep 2012 15:12

When I replied earlier, I didn't know you were out in Bolivia. My remarks were aimed at someone working at home with the usual tools and facilities. In your circumstances, I would be tempted to leave it alone and get it to mechanic or BMW dealer when you can. Just ride around the problem for now.

Given that this is not just a simple case of bleeding the rear line and caliper (thank you BMW), I would sooner do that than risk losing the brake completely.

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