Rear brake proportional valve (or ABS unit) issue - BMW K1600 Forum : BMW K1600 GT and GTL Forums
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post #1 of 16 Old 09-16-2019, 04:47 AM Thread Starter
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Rear brake proportional valve (or ABS unit) issue

Hi all,

I've searched a lot on the forum for the specific issue I have but haven't found anything similar, so I'm asking for your help in case anyone faced it or has any ideas:

- rear brake pads changed last week in a hurry due to late delivery from the dealer (and a long trip already scheduled for last weekend). Therefore I didn't have time to do it as per my previous procedures: open the bleeder, connect it to a hose and then to a small bottle of brake fluid, open the bleeder while pushing back the rear caliper piston in order to avoid old brake fluid going back in the master cylinder and ABS unit, causing potential damage. I use this practice after a nasty episode about 15 years ago when on a previous car the ABS unit got damaged due to the old brake fluid being pushed from the calipers to the unit;
- unfortunately, due to the rush, I didn't wash the visible part of the piston either (with a brush and brake fluid) for avoiding potential rings damage or piston seize due to dirty piston (though the exposed area of the piston looked somehow clean);

- and now the nasty part: during the 600 miles trip from the weekend the rear brake showed some abnormal behaviour. One of the issues is that the travel is longer than usual; initially if I pushed the pedal once more the travel seemded shorter, which could mean some air in the system. But after a few more miles the second/third push on the pedal didn't improve anything anymore. And, the most important, it seems that the proportional valve stopped working: while using both the front and the rear brakes I do not feel anymore the 'click' in the pedal which happened at every combined usage of the brakes. It clicks sometimes but randomly (every 6-8 usage of both brakes let's say) which makes me think the proportional valve in the ABS or some other valve got partially stuck/damaged.

I checked the rear ABS and it works properly under hard rear braking. Also while applying only the front brake the force in the lever but also the brake force itself are normal, so I assume the front/rear combined braking still works fine. I noticed no rear brake dragging (no heat issues).

As next action I'm planning a fluid change (~1 year old Motul currently) and use GS911 for additional bleeding but I doubt it will solve the problem. By default the GT seems to have an unusual long brake pedal travel (it can be slightly adjusted) but now, when it became even longer, it's disturbing. Even if I get used to the longer pedal travel I am not sure it will behave consistently (sometimes it still seems to 'bite' in a higher position) and it creates a feeling of 'not very safe' when I know it doesn't work as designed . Most of my miles are hard-riding on curvy roads and I use heavily the rear brake, so I need to solve it somehow...

Thanks in advance for any ideas.
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post #2 of 16 Old 09-16-2019, 04:51 AM
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If there is a serious issue with the ABS unit, you should see a code.
Go back to where you started and do it again.

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post #3 of 16 Old 09-16-2019, 08:10 AM
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From everything the OP has written I'd say you don't have a problem. You wrote: "while using both the front and the rear brakes I do not feel anymore the 'click' in the pedal which happened at every combined usage of the brakes." If you're applying the front and rear brake at the same time, don't.

I've learned the hard way to NEVER perform ANY heavy maintenance prior to a planned long ride. I define 'heavy maintenance' as tires, brake pads, bleeding system, additional wiring/lighting, etc. Basically anything other than routine cleaning/washing. If you do and your luck is like mine you'll answer to "Murphy" every time!

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Last edited by Gunnert; 09-16-2019 at 08:14 AM.
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post #4 of 16 Old 09-16-2019, 09:01 AM
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There's absolutely no reason to crack the bleeder open during a pad change. Just push the pads back and install the new ones. Pump the break lever/pedal up after the install and your good to go.
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post #5 of 16 Old 09-16-2019, 09:04 AM
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Rear brake pad replacement should take less than 10 minutes and won't affect anything. Ride the bike and stop worrying
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post #6 of 16 Old 09-16-2019, 09:12 AM
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Quote:
Originally Posted by cocoon View Post
Even if I get used to the longer pedal travel I am not sure it will behave consistently (sometimes it still seems to 'bite' in a higher position) and it creates a feeling of 'not very safe' when I know it doesn't work as designed . Most of my miles are hard-riding on curvy roads and I use heavily the rear brake, so I need to solve it somehow...
Coming from an RT, the rear brake action on the K bike bothered me too but @Gunnert and others here in various threads explained how the linked brakes work on the bike. If you are relying on or using your rear brake heavily in the curvy roads, you will get in trouble quickly with the K bike. I now exclusively use the front brake lever and only use the rear lever when under 5 mph or so (slow speed stuff). There are several threads on this subject but it sounds like you are experiencing what I also thought was poor rear brake action but is really normal for the bike due to how the front and rear are linked.
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post #7 of 16 Old 09-16-2019, 10:37 AM
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There is no reason to open the bleeder valve during a pad change. As stated, go back and do the whole fluid bleed change correctly.
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post #8 of 16 Old 09-17-2019, 05:35 AM Thread Starter
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Thanks all for the replies and suggestions.

Just as clarification - I also try to avoid any maintenance right before long trips but this was an exception - the brake pads were delivered late and also the trip from the weekend was decided last minute based on a weather forecast improvement

Regarding the valve opening while pressing in the caliper piston: as mentioned, many years ago I had a nasty issue due to that on a car - the ABS unit got damaged due to 'back pressure' pushed in the unit and, potentially, due to old fluid (dirty?!) entering the ABS unit and messing the tiny/sensible valves inside. And I also saw such recommendations in some maintenance manuals (for cars) - applying such procedure should not hurt in any case.

And for the actual problem I'm facing:

- the proportional valve definitely is not behaving as it used to - I meant when the pedal is already pressed, if I pull also the front lever I do not sense the 'click' anymore - it happens only randomly (once every 6-8 pushes on the pedal);
- about using heavily the rear brake - I use it only for 'trail braking' while riding on curvy roads. Using the front brake in such situations will cause nose dive and also bike rising from leaning position.


I'll run also a diag with GS911 just to make sure the ABS unit is not reporting any issue; though I am sure something happened - in 40.000miles riding this bike I got a good idea on how its brake works and I know it's not behaving normally. The main issue is with the extended pedal travel until the rear brake bites - I'll see if things improve after a fluid change and bleeding.
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post #9 of 16 Old 09-17-2019, 06:24 AM
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A few possibilities occur to me.

It could be pad 'knock-back' if the disc is slightly out of true. Or maybe the caliper not sliding cleanly on the pins - it's a twin piston sliding caliper, so uneven wear on the old pads would provide witness.

Otherwise, assuming disc scoring is minimal, then a good brake bleed should sort the issue IMHO.

However we have all seen/heard of a few brake 'feel' mysteries over the years - the last I came across personally was an almost new F700GS I had on loan. Knowing I had complained, the dealer later told me it proved to be a rear master cylinder failure; I know not what precisely though as, being under warranty, I don't expect they looked too hard. It behaved much like the OP describes.
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post #10 of 16 Old 09-17-2019, 06:31 AM Thread Starter
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Originally Posted by squibb View Post
A few possibilities occur to me.

It could be pad 'knock-back' if the disc is slightly out of true. Or maybe the caliper not sliding cleanly on the pins - it's a twin piston sliding caliper, so uneven wear on the old pads would provide witness.

Otherwise, assuming disc scoring is minimal, then a good brake bleed should sort the issue IMHO.

However we have all seen/heard of a few brake 'feel' mysteries over the years - the last I came across personally was an almost new F700GS I had on loan. Knowing I had complained, the dealer later told me it proved to be a rear master cylinder failure; I know not what precisely though as, being under warranty, I don't expect they looked too hard. It behaved much like the OP describes.

Thanks for suggestions. The disc is fine (not warped, thickness still in limits) and the caliper slides also fine.


I'll try soon the complete bleeding and I'll see after that. Honestly I was also thinking of the rear master cylinder for potential culprit as second suspect after the ABS unit. At least the master cylinder is much cheaper than the ABS . But who knows, maybe I'm lucky and just a bleeding will solve it, I'll post the result after I find some time to work on the bike

Last edited by cocoon; 09-17-2019 at 06:33 AM.
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