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'03 1150RTP ABS Unit dumping fluid from drain hose.


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We have a 2003 1150RTP that keeps throwing the abs warning lights. Alt flashing at 1hz would indicate low internal reservoir fluid. I pulled the tank back in Nov and filled both of them to the limit. A few weeks later the lights came back on. I went and filled them last weekend to the limit (they appeared empty again), and today while parked on the centerstand we noticed that there was fluid leaking from one of the drain tubes. A closer look showed it was brake fluid. Does anyone have any ideas on why the internal reservoir would overflow and drain out? Front and rear pads are close to being worn if that matters at all.

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Evening KER


That is a difficult diagnosis over the Internet without a lot more info.


With fluid coming out of the vent hose it sounds like it was overfilled. Or for some reason the fluid in the reservoir is gaining height.


Assuming you didn't overfill one of the reservoirs my first thought is you have a loose wheel bearing or crown bearing allowing a wobbling wheel to kick the brake pads back into one of the calipers.


No chance you have installed a new front tire since filling it last? If so, just pushing the pads back into the front calipers to remove a wheel could be the reason for fluid in the vent hose. (see overfilling below)


If no loose wheel or crown bearing & you didn't overfill it last time then other possibilities are:


--Excess moisture in the brake fluid causing steam pockets under very heavy prolonged braking.


--Or, possibility one of the servo pumps is running all the time & not shutting down.


--Or, somehow air is being drawn in at a servo pump seal or controller seal (long shot this one)


When filling the controller reservoirs you need to take into account the brake pad wear. If you fill to the high limit with worn brake pads you are overfilling the system. (that is the idea behind the special BMW brake re-setting tool as it holds the caliper pistons at the correct position regardless of brake pad wear).


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Nathan Margolis

I would start with the brake flush by the book. There are a few good step by step procedure on the Internet you can find on this. Are you are ware there are 11 bleeder valves and certain sequence to do it in? If you need the websites and manuals, contact me on this forum and I will be glad to email you the PDF files.


Nathan Margolis

'13 BMW R1200RT

'04 BMW R1100S

'02 BMW R1150RT (sold for the '13)

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  • 1 year later...

Still having the issue but now Im having it with about 3 of my bikes now. All brake function is normal, all fluid levels are filled to normal levels (not over filled). Fluid only appears to be dripping out when the bike is parked, on a level surface, on the side-stand or center-stand. I have flushed the abs system with new fluid as well. Whats odd is that it appears to leak until it finally will trip the internal abs warning that the fluid is low so it doesnt seem to matter if its completely full or totally empty. I dont know what to do anymore with these bikes......

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You always have interesting problems.

Is there a chance the brake fluid is contaminated or defective? Perhaps trying a different brand? Were you using a fresh bottle of Dot 4?

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Morning KER


Are you sure it is brake fluid dripping?


There is a water drain & fuel overflow drain in that group of hoses.


If "for sure" brake fluid then you are either fighting with incorrect brake fluid, or fluid boiling, or fluid aeration, or SOMETHING forcing the fluid back & UP through the cover vents.


Or for some reason the brake fluid in the resiviours is running up hill & out the venting system.


If no sign of brake pad kick back (like loose wheel bearings or loose crown bearing) then maybe a servo pump issue or ????


You aren't laying the bike on it's side when you park it are you?


When servicing the ABS system are you properly shimming the caliper pistons back into the calipers when filling the ABS controller resiviours.




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I like DR's idea about fluid boiling up & out. Do you think someone could be riding the rear brake? You should be able to trace the leaking line back to the controller and see whether it's coming from the front or rear overflow line. Might be interesting.

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