85 K100 Basic - rear master cylinder not receiving fluid from reservoir

[rawhites]

Bike sat for 30 years....when I went to bleed out & refill the rear brake, I found that the rear master cylinder isn't receiving fluid from the reservoir. The cylinder piston seems to action/work well, driving old fluid out of the bleed nipple. And new fluid can pass through the line down from the reservoir (when detached). But I cannot push new fluid into the system. So it seem there's a blockage internal to the master cylinder....either the receiving nipple is blocked or something else is internally.

I saw other threads about struggles replacing the 13mm Magura (my luck to have the same). No stock on euromotoelectrics or the boneyard.

Two questions:
- Anyone found a source/replacement for these? Whether the 13mm or other sizes that work decently well?
- Anyone have a video on how to do the 13mm rebuild? Happy to do the work, but Haynes is of no help.

Thank you!

 

[richo2006]

I just looked at the Max parts fiche and they show new master cylinder and rebuild kit for the (D=13mm MAGURA).

Regards,
Richard in Commerce Township MI

 

[6322]

EME is a good source for K parts:

High quality and high pressure brake lines for BMW R Airhead R45, R50, R60, R65, R75, R80, R90, R100

High-Quality, Low-Priced Motorcycle Parts: Specializing in Starting, Charging, Cooling, Engine Management & Filtration. BMW Motorcycle Parts, BMW ALL Motorcycle MODELS, AIRHEAD, BMW OILHEAD, BMW HEXHEAD, R100, R80 R75, R50, R60, R65, R45, R90, R1100, R850, R1150, R1200, K100, K75, K1100, K1200...

www.euromotoelectrics.com

 

[rawhites]

I just looked at the Max parts fiche and they show new master cylinder and rebuild kit for the (D=13mm MAGURA).

Regards,
Richard in Commerce Township MI

Thanks for spotting one, Richard! Just ordered.

 

[rawhites]

Epilogue....the MAX fiche was not up-to-date: they have no 13mm rear master cylinders and can't source one.

So I'm back to the drawing board. Anyone have a video for rebuilding these? Or know of any substitutes?

Rob

 

[rawhites]

Anyone know if this 16mm "upgrade" would do the trick? Per the EME application table, it would fit the ECE version of 85 K100 Basic, so I assume it would be okay on my (presumably) USA version?
https://www.euromotoelectrics.com/product-p/bmcylinder142u.htm

 

[6322]

I replaced my original with EME’s 16mm on my ‘85 K100RS w/EML sidecar and it works great. Also updated original rubber brake lines with Spiegler stainless lines. The results were positive. Better brake feel, more firm, and better braking.

 

[rawhites]

Thanks, Gary! I'll look into that option....sounds better than a $275 rebuild with Apply Hydraulics.
Did you replace the lines because the bigger cylinder required a stronger line? Or will the original 13mm cylinder lines work?

 

[6322]

Thanks, Gary! I'll look into that option....sounds better than a $275 rebuild with Apply Hydraulics.
Did you replace the lines because the bigger cylinder required a stronger line? Or will the original 13mm cylinder lines work?

I replaced the lines because they were thirty plus years old and something new, something better (stainless lines), was now available. The original rubber lines will work if they are as good as new.

 

[98lee]

Thanks, Gary! I'll look into that option....sounds better than a $275 rebuild with Apply Hydraulics.
Did you replace the lines because the bigger cylinder required a stronger line? Or will the original 13mm cylinder lines work?

Rob,

The larger diameter master cylinder (16mm) will result in LOWER line pressure for a given pedal input. It will result in less pedal travel. It will give a more firm pedal feel. All of these things are things that I prefer.

It will result in more pedal input pressure for a given stopping effect (you will have to press harder). How much harder is the question. Since your rear brakes do a smaller proportion of the stopping, I’m seriously considering
changing from the 13mm on the rear of my K75S (which works just fine) to the 16mm master to gain the possible advantages listed above.

 

[rawhites]

Thanks, Lee! I’m a beggar not a chooser at this stage of my project, so the 16mm sounds like a great option!

 

[richo2006]

Rob,

Have you dissembled your existing master cylinder and tried to clean the fluid inlet? If you want I can post some pictures to show how to take it apart.

Regards,

Richard in Commerce Township MI

 

[rawhites]

Hi Richard - sure, happy to give it a shot. But couldn’t find any guidance online. Pictures would help!

 

[richo2006]

Rob,

Remove the rubber boot from the cylinder. Then follow instructions as shown. You may be able to clean, dry and reassemble the master cylinder and it may work just fine. When cleaning I use my wife's toothbrush, soap and warm water. I use a thin coat of silicon based brake assembly lube when reassembling brake cylinders.




Loosen this screw


Hold as shown to keep piston from flying out of cylinder and remove the screew


remove piston from cylinder



Clean as necessary and reassemble.

 

[rawhites]

Perfect - thank you! I'll give it a shot.

 

[pauls1150]

Does the wife know about it yet....?

 

[richo2006]

Not yet Paul. There's nothing wrong with a little passive aggressiveness.

 

[AntonLargiader]

If it's not filling, start by looking online for some diagrams of how a master works.

The primary seal (to the right in the pic above) needs to retract beyond the small equalization port, but it mostly refills the pressure side of the system by letting fluid going past the seal (from the area between the two seals) during retraction. The fill port (larger of the two holes) keeps that area full of fluid for this purpose.

If you have a lot of crud in that fill chamber between the seals, or in front of the primary seal, this refill action may not work.

 

[rawhites]

If it's not filling, start by looking online for some diagrams of how a master works.

The primary seal (to the right in the pic above) needs to retract beyond the small equalization port, but it mostly refills the pressure side of the system by letting fluid going past the seal (from the area between the two seals) during retraction. The fill port (larger of the two holes) keeps that area full of fluid for this purpose.

If you have a lot of crud in that fill chamber between the seals, or in front of the primary seal, this refill action may not work.

Thanks, Anton - great advice. Any visuals/videos on getting that plastic inlet piece out of the top? I need to pull it out in order to inspect & clean those ports.

 

[AntonLargiader]

It will pry out, but it's easy to break. You can buy new ones from EME.