K75S rear master cylinder rebuild

[kbikeMIA]

1986 K bike rear master cylinder is leaking. I have owned the bike since 1988, and this is the first I recall needing to service this bit. I Obtained an OEM kit, and following the Haynes diagram and online parts fiche illustrations, I can not determine if the piston and spring is retained in the housing by a circlip, or when installed the master cylinder, pedal assembly and adjuster properly set, keeps all together, OR the crud that has built up over 30 years plus is keeping the piston from being extracted. The after market repair guide says to simply "look around for a retaining pin or clip". All diagrams I have viewed do not illustrate a retainer.

Probably should have opted for a complete replacement at this rate.

Any clues? kbikeMIA

[rbm]

Grub screw on the side of the MC retains the piston in the cylinder.  Undo the screw and the piston will be able to be removed.

[kbikeMIA]

Robert,
Absolutely correct! The screw is finished like the MC body and appears to be seamless and part of the casting. Fooled me!
Thanks for the info.

Jim,
Miami

[hillbillyjoe]

Hi Robert & Jim, I just rebuilt the master cylinder on my new to me K75s, and it won't pump up. The bore seems fine, and all the lines flow freely. The spring in the kit has a small end and a larger end, which one goes toward the piston? or does it make a difference? I've tried the reverse flush and it doesn't work, I've tried a vacuum, and that doesn't work. The kit was a magura from beemerboneyard(aren't BMWs prices ridiculous?) Any suggestions? I'm at my wits end and don't know where to turn next. I'd hate to buy a new master cylinder and have the same problem. BTW I had the same problem removing the piston LOL. Any suggestions would be appreciated. Thanks in advance for any help, I'd rather be riding than wrenching. See ya Phillip

[rbm]

Philip,  When you say it won't pump up, I guess you mean that you don't feel resistance against the brake pedal which indicates air still trapped in the system or air entering the system through a leak. 

Small end of the spring goes towards the piston; the large end towards the rubber dust protector.

First off, is this K75S an ABS bike or non-ABS?  Were there any other parts replaced or restored on the rear braking system  in addition to restoring the rear MC, such as brake lines or caliper pistons?

[kbikeMIA]

Phillip, Robert,  All good info for tomorrows wrap up, hopefully. I will post the results of efforts. Thanks all. JS

[hillbillyjoe]

Robert, correct on the no resistance, it's a '93 w/abs, & nothing else was changed.The rear brake was working when I bought it, really mushy feeling, but it did work. By the time I got home, maybe 50 mi., no rear brake & the abs light & red triangle light flashing. On closer inspection, noticed quite a bit of corrosion under dust boot. When I took the master cylinder apart I was expecting a lot of pitting in the bore, but there wasn't any. Took some 400 wet or dry sandpaper wetted w/brake fluid & it shined up real nice. Didn't bench bleed(didn't know how) and couldn't get any pedal. Took it apart again & attempted to fill mc manually, still no pedal. Took a large vets syringe filled w/brake fluid & a clear hose, attempted to reverse flush the system, at abs modulator & at caliper & could get nothing to flow, pedal partially open & fully closed. Tried to elimanate a collapsed hose by taking them lose one at a time, & fluid flows freely, but with everything hooked up, nothing flows, fluid or air. Sorry for the long winded reply, I'm taking it apart again in the a.m. & try to determine what's up, & see which way the piston spring is installed. Since it had rear brake to start with, I kinda ruled out collapsing hose. We'll see how it goes. Thanks for reply, sorry to hijack this thread, but it seemed relevant. I'll post results or further questions if I find anything out. See ya, Phillip

[rbm]

I've no experience bleeding ABS I systems; all my K's have been non-ABS.  The process for ABS is more complex.  I've read that the ABS modulators can seize but since you had rear brakes before, this possibility might be eliminated.  Have you read through some of the threads on ABS bleeding?
http://www.motobrick.com/index.php/topic,1135.msg14430.html#msg14430
http://www.motobrick.com/index.php/topic,6828.msg46804.html#msg46804
http://www.motobrick.com/index.php/topic,5709.msg36607.html#msg36607
http://www.k11og.org/forum/viewtopic.php?t=9690&highlight=bleed+rear+abs

[kbikeMIA]

With the MC on the bench, the grub screw was easy to ID, thank you Robert, and remove. The piston was still resisting 28 years of comfort in the hole, but was persuaded out with a bit of effort. Once apart the MC bore was surprisingly with out any pitting. Used the method previously posted using 0000 steel wool saturated in brake cleaner then a fresh 0000 pad with brake fluid to smooth the bore, it cleaned up excellent. The Beemer shop special order parts kit turned out to be unusable, at great expense, as the diameter was .5mm smaller than the original, and the rubber dust cover was too small to fit. On inspection the original bits were still soft and pliable,cleaned with brake cleaner primed with brake fluid and re installed.  Applied a very small bit of RTV sealant to the rubber boot and secured in place with a tiewrap. This non-ABS system was not an effort to bleed. Used the pump three times, unlock then lock the bleed screw routine. Atfer 25 reps, no air in the system and a solid pedal. 1.5 hrs later, back on the road.

Thanks for all the tips and guidance,

Jim Smith kbikeMIA

[rbm]

Way to go Jim.  Sounds like your BMW shop probably supplied you with a 12mm kit and you have a 13mm MC fitted.  There could have been a previous owner at some stage who replaced the cylinder.  It's a bit late now but the size of the MC is stamped on the side.

[kbikeMIA]

Robert, It is most time an educational experience, usually all good and I enjoy working on the bike.
I am pleased to have joined the Motobrick community, and received the feedback.
Thanks again and more soon.

Jim

[hillbillyjoe]

Jim, glad you got yours fixed, sorry for not getting back sooner, but my computer threw a scrap iron fit.(hillbilly term for, quit working, $100 later & I'm back) Robert, I read the links you suggested & tried the ones that sounded close to my problem, to no avail. My mc is marked 12, but seems to fit a little loose. I guess I'll take the plunge & buy a overpriced mc & see what happens. Thanks for the help, see ya, Phillip

[K75RT Keith]

I think in the Liberry is a listing of alternate parts.  One is for the rear M/C.  If it is a 12mm , there are a few options even on flea bay that are much less expensive than buying from BMW.

[hillbillyjoe]

Robert & Jim, Finally, rear brakes. Thanks so much for suggestions & links to possible fixes. I wound up replacing mc, after finding out the new rebuild kit piston fit a little loose in the bore. I did have to bypass abs modulator, by taking line directly to caliper. Took quite a while to get all the air out, but it finally works. Thanks for the help. I'd love to post some pictures, but I'm not that computer savvy, again thanks. See ya, Phillip.

[kbikeMIA]

Well done Phillip! I am working on the next challenge, fuel level sender malfunction. More soon.

Jim

[hillbillyjoe]

Thanks Jim, I guess my next project is getting the abs light to quit blinking. I don't know if by bypassing the modulator will affect the reset prosedure or not, we'll see. The red triangle light is also blinking, I hope it resets also. See ya, Phillip

[Motorhobo]

My guess would be that bypassing the modulator would ensure that the ABS light blinks unless you take some action to disable the light. I would think the missing modulator would register as a fault, and the fault will persist even after reset. THe ABS brain is probably expecting some feedback from the modulator that it's not getting. Either way -- the cheap, easy fix is a strip of black electrical tape over the blinking light. You wouldn't be the first.

Now that you have brakes, maybe you should hook up the modulator again and see if you can bleed it out and get it working. I haven't seen any indication in your posts that the modulator was the cause of any problem. I personally would rather ride with ABS than not. If you do that, don't forget that you have to get the bike on the road at a reasonable speed after the reset before the light stops flashing. I forgot that little fact once and beat myself about the head wondering why the light wasn't going out after repeated resets. Finally, I gave up and resigned myself to the flashing light and decided to go for a ride. At about 20mph, the light stopped flashing and hasn't flashed since, a year later.

[hillbillyjoe]

  Motorhobo, thanks for the info, I kinda suspicioned that would be the case. I will try hooking the abs modulator again. Right now I'm in the process of getting all the fluids replaced, & the splines greased. Will probably hook it up then. I'd rather have the abs if possible. Again, thanks. See ya, Phillip