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Hello all, I discovered my 2003 K1200GT is venting fluid from one of the four drain tubes that exit down by the rear wheel/centerstand. I thought it was antifreeze at first but was surprised to find it is a tube originating from the rear brake reservoir (integral ABS). In the attached photos of the reservoir, the source is the hose circled in red.

The fluid is near the MAX mark, but is this normal? I thought brake fluid was always supposed to be sealed and was quite surprised to see that the reservoir is vented to the open air, BTW. (?) The parts fiche lists a "lid with vent hose" (photo attached) so it seems that's planned?!

The system was bled and fluid replaced by the local shop almost a year ago, but sadly very few miles.
 

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Can you expand on what was happening when the venting occurred? If you had the rear wheel off for example, pushing the pads back to get the caliper clear of the brake disk Would push fluid back up the lines and a full reservoir would vent.
 

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Can you expand on what was happening when the venting occurred? If you had the rear wheel off for example, pushing the pads back to get the caliper clear of the brake disk Would push fluid back up the lines and a full reservoir would vent.
Sorry, should have been more specific.

The rear wheel had not been disturbed. The front had been removed and reinstalled (front pads were pushed back some to facilitate install). I noted the venting while running the engine and applying brakes (hard) with the front lever while compressing the fork to align the front end. The fork legs had been removed for new seals.
 

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Sorry, should have been more specific.

The rear wheel had not been disturbed. The front had been removed and reinstalled (front pads were pushed back some to facilitate install). I noted the venting while running the engine and applying brakes (hard) with the front lever while compressing the fork to align the front end. The fork legs had been removed for new seals.
Given this added info, what you saw is somewhat normal - although I am not on-site to confirm all work done and all symptoms.

BY DESIGN ALL BMW "K1200" and Boxers R1150/R1200 between 2002-2006 in USA (or 2001-2006 for other countries) use a similar ABS modulator design that is servo assisted:
1) the Front "Control circuit" of brake fluid is between handlebar Master and ABS modulator - there is NO venting but expansion is allowed inside by classic rubber boot inside Master cylinder at handlebar.

For the Rear "Control-circuit" , the master-cylinder cannot have a rubber boot, so there is venting/expansion from the most external section of the white reservoir showed in your picture. This is the reason why there is a small black hose between reservoir out-most section and Rear brake master.


2) the "Wheel circuits" of brake fluid are between the ABS modulator and each wheel calipers - venting and brake pads wear compensation is made by venting 2 reservoirs (1 for rear circuit and 1 for front circuit).

On Most Boxers models, both small reservoir (front and rear) are embedded into the top of the ABS modulator - on these models the ABS modulator is mounted Horizontally in frame. On K1200RS/GT and also on some K1200LT (2002-2004) the ABS modulator was mounted vertically instead - in such position they could NOT use the top reservoirs so they used a separate 2 sections white / opaque reservoir on right side (below seat).

As the brake pads wear, the brake fluid level goes toward MIN mark, When brake pads are new, the fluid level should be adjusted to MAX mark. Whenever the pistons of Caliper(s) are opened up (to remove caliper / wheel) this will makes the level go up momentarely in the specific (front or rear) reservoir.

If the fluid level was a bit too high and not proportional to brake pad thickness, there could be an overflow for one or both venting hoses. If your bike is 100% stock, these 2 small black venting hoses go up a bit from the white reservoir (tied to sub-frame under seat), then they curve down together (passing near battery box) to end below rear swing-arm to vent toward the ground (in case of overflow).
 

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Discussion Starter #5
John, thank you so much for your insight. My mind is blown with this. How can brake fluid—which absorbs water and is supposed to be flushed periodically for this reason—be vented to atmosphere?

I’m glad nothing is broken on my K, though.
 

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John, thank you so much for your insight. My mind is blown with this. How can brake fluid—which absorbs water and is supposed to be flushed periodically for this reason—be vented to atmosphere?

I’m glad nothing is broken on my K, though.
To compensate for the venting issue, the maintenance schedule requirements are Yearly DOT4 fluid bleeding for the "Wheel circuits". The other circuits ( Control circuit) are sealed more like a regular non-ABS system - these should be done every 2 years on most models.

These servos assisted ABS system are complex and strange - I am not the only one with this opinion. You should see the mind boggling complexity internals of these ABS modulators - German over engineering at its best (or worst in this case).

A company in the USA who specialize in the repair of automotive electronics and ABS module (AUDI , Mecedez, BMW...) once said something similar to my above opinion - and I think they were being polite ;-)

Having followed the history of these (and the many failures) on 3 busy Forums since 2002, I would say the maintenance (yearly bleeding) is one important part of the equation to make these Modulator last. We have found internal corrosion on bike that sat for a long time unused.

In my opinion, another important factor is the usage: a lot of city driving will wear out the electric sevo motors faster. Finally, to keep the system internals moving, I do force the ABS mode to activate 4 or 5 times during the summer months (when we can ride here) - this can be done fairly easily on a gravel road - be VERY CAREFUL as this can cause a crash / fall if your ABS mode is defective.
 

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I see a rubber brake line on the OP's pic.:glasses Should all be replaced with Teflon SS braided lines.

Also.....no need to pull the forks to replace the seals.I'd say an hour work max to replace seals without pulling the forks.

I bleed my system yearly.And stick the brake fluid moisture tester into the old fluid at each bleed point.Never detected any moisture at any of them.
 

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I see a rubber brake line on the OP's pic.:glasses Should all be replaced with Teflon SS braided lines.

Also.....no need to pull the forks to replace the seals.I'd say an hour work max to replace seals without pulling the forks.

I bleed my system yearly.And stick the brake fluid moisture tester into the old fluid at each bleed point.Never detected any moisture at any of them.
Very good point about the OEM factory rubber brake hose. Thanks!

In my recommendations (in previous post) I should have included the replacement of these original rubber brake hoses when they have reached 10 years (on average). Past 10 to 12 years, the failure rate has been higher.

In addition, we have often found small rubber debris when bleeding bike having these old black rubber brake hoses - after a long time they breakdown from the inside. These debris will eventually kill the modulator internals( worst case) or will increase probability of failure.
 

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Another thing to check on the reservoirs under the seat is the flat rubber washer under the plastic threaded filler point. I was at a loss why there was water floating on top of the brake fluid. After a fluid replacement, the moisture was back again a few weeks (and bike cleans) later. The washer had deteriorated and stretched considerably allowing water to enter the rear reservoir. There are plenty of cheap aftermarket seals available to fix this.
 

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Another thing to check on the reservoirs under the seat is the flat rubber washer under the plastic threaded filler point. I was at a loss why there was water floating on top of the brake fluid. After a fluid replacement, the moisture was back again a few weeks (and bike cleans) later. The washer had deteriorated and stretched considerably allowing water to enter the rear reservoir. There are plenty of cheap aftermarket seals available to fix this.
Yeah.....as found on a GS.Someone had installed both rubber washers on the same side of the filler cap.On the upside so no sealing at the reservoir.

Same caps but the reservoir is in the modulator on a GS.I installed new brake pads expecting the excess fluid to drain down to ground via the vent hose.Found most of it on top of the modulator.:sad

Of course one of the washers was trashed from being squeezed against the other.Quick fix was an Oring.

I don't have a clue how long that had been going on.All I know is that the owner told me the fluid had been changed recently and that there was no need to flush it so I did nothing but the necessary caliper pistons cleaning,new pads and new rear rotor.....!:crying

iABS system started acting up a year(?) later.Dangerously acting up so I removed it.Stuck the brake fluid moisture tester in the reservoirs as I worked on the iABSectomy and moisture tester lit up like a Christmas tree at over 5% moisture content in both reservoirs.
 
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