No Rear Brakes 94 K75RT

[The Mighty Gryphon]

My K75RT ABS has had a leaky rear master cylinder since I got it last year.  Put on a used master cylinder which was better, but still leaked due to pitting in the bore.

Sent out the original to Apple Hydraulics who advertise BMW motorcycle master cylinder sleeving.  Reinstalled sleeved master cylinder with a Magura rebuild kit from Beemer Boneyard.   Put it all together and the pedal was mushy so I had a set of stainless braided brake lines made.  After reassembling everything and bleeding about a half quart of brake fluid through the system the @#$%^&* pedal now just goes right to the stop and absolutely no braking force is generated. 

When I bleed I get no air in the fluid once the original bubbles were removed.  If I open the bleeders I can pump fluid out of the system and there is no air in it.  After nearly 10 hours screwing around with this thing I have no idea what to do next.  I have checked everything from the boot to the banjo fittings and caliper and there are no leaks anywhere.  I have had it apart three times and and everything looks perfect, and yet it acts like there is no seal on the piston.

[koapono]

it is often the simple things that stymie us.........
i would remove brake line at M/C, place finger over discharge port and depress pedal. Is there pressure/resistance or does pedal move without displacing fluid? piston to bore clearance may be excessive, close enough to move fluid through system (discharge at bleeder) but not provide the hydraulic pressure required to apply the brakes.
seems you've checked everything else.
g'luck

[Laitch]

I think kaopono is on the right track. Also, if no fluid is leaking at the boot then I'd investigate the possibility that air is being drawn into the system on the intake side, maybe a leaking reservoir seal or intake tube. I lost braking in a Honda Civic in congested and intermittently, crazily speeding traffic on the Cross-Bronx expressway late afternoon the Friday of one Labor Day weekend. :mbird  I made it to the New Jersey Turnpike rest area across the George Washington Bridge by using the emergency brake. After I had a cup of coffee and a few deep breaths, I opened the hood. The brake fluid reservoir cap was loose and had almost fallen into the engine compartment. I tightened it up, pumped up the brakes and headed to Miami without further interruptions.

[mw074]

There are 2 different size rebuild kits. Just a thought, check that you have the correct one.

[The Mighty Gryphon]

Will be diving back in this afternoon.  Pretty sure I have the right rebuild kit.  The master cylinder is marked 12mm and that is what the rebuild kit is for.  Don't think there is a smaller one. 

Good tip on the intake leak.  I had to remove the hose to the reservoir to send in the cylinder for sleeving.   Pushed the hose back onto the barb and the joint doesn't leak brake fluid, but it may be sucking some air.  Will pick up a screw clamp today.

Will also be checking the bore diameter of the master cylinder for size.  Hard to believe an outfit that has been doing this work for years would screw something like that up, but who knows.

Will post an update later today.

[The Mighty Gryphon]

One other thought.  Does anyone know what the correct routing of the reservoir hose is?  Seems like it gets pinched between the footpeg plate and the transmission.  I can't find a way to get it out that doesn't wind up pinching the damn thing.  I wonder if that is the problem.

[K1300S]

Did you bench bleed the master cyl before installing? Sounds to me like there is still a big bubble of air in it. 

[The Mighty Gryphon]

Did you bench bleed the master cyl before installing? Sounds to me like there is still a big bubble of air in it.

No, I just pumped a shitload of brake fluid through it.  Most of that fluid came out with no air bubbles.  I suppose I can try that when I tear it down again this afternoon. 

[K1300S]

I have found on many cars that bench bleeds are needed.  Regular bleeding won't get all the air out of a dry master

[The Mighty Gryphon]

Will give it a try. 

[Laitch]

OK. After actually noticing the giant "ABS" letters in one of your images, I realized I'm out of my depth here, never having worked on ABS brakes myself. What I do understand now, courtesy of MOTOBRICK.COM links compiled by rbm, is that bleeding can be a different kettle of fish. Maybe you've done ABS procedure previously, Gryph, but here's a link to his contribution in a brake thread anyway.
http://www.motobrick.com/index.php/topic,7239.msg50201.html#msg50201

[The Mighty Gryphon]

Update and possible final post on this problem.

I took the rebuilt master cylinder and did a "bench bleed".   Put it back on the bike and still no pedal.  Now have brake fluid coming out of the plastic cap that the reservoir attaches to.  Took it off the bike and with all the self control I could muster refrained from throwing it as far as I could into the woods behind the house.  Pulled the cap and put a bead of blue Permatex around the mating surface and stuck it back in.  That cylinder is going into the spares inventory with the hope I never have to put it on a bike again.

Took the master cylinder that had been on the bike while the original was being resleeved and honed out the corrosion in the bore that was causing the mushy pedal and slight leakage.  Installed on the bike and I now have rear brakes.  With winter coming soon, it will probably be next spring before I know if there is any leakage past the seals, but for now it feels pretty good.

[Laitch]

Another success story, more or less! Your self-control is admirable, MG. You've affirmed what Calvin Coolidge (that famous Vermonter who lived most of his life in Massachusetts) frequently asserted—persistence is everything. I guess this means the Apple Hyd. rebuild won't get a USPS ride back to Calverton with WTF painted on it in scream green.

[K1300S]

woo hoo!  brakes are good!

in the old bmw car community, our phrase for parts like that is, "toss it over the fence into the neighbors pool".

[The Mighty Gryphon]

Yeah, way too much time on that job.  Glad it's behind me, but there is a lot of money tied up in that master cylinder, so I imagine I'll be trying it again some time hoping that the Permatex has fixed the leak.   With two more bikes using the same part, there will probably be an opportunity at some future point.

Calvin Coolidge, as a libertarian, the more I learn of his life and presidency the more I admire him and think he may have been the best president of the 20th century.