Hi all, - I have a case which, to me, seems to be deeper and more difficult to solve than I thought, - maybe due to missing information from different webshops and manufacturers.

I am rebuilding my 1990 K100RS-4V into a kind of a street fighter, with (among a lot more) clip on handlebars and a new front master brake cylinder.

MY PROBLEM IS TO JUDGE WHICH PRODUCTS ARE COMPATIBLE WITH MY BIKE. And I hope to get some qualified feed back from you guys.

I'll take you through what I know, and what I still don't get:

I want to buy a master cylinder like these two:
Magura HC1: https://www.magura.com/en/components/powersports/radialmastercylinders/productdetailpage/?p=1851
or
Brembo 19RCS: http://www.carpimoto.com/en-NO/50719_RCS19PKB-Radial-Brake-Master-Cylinder-Brembo-19RCS-Premium-Kit-Black.htm

My problem is that I don't know if I can use any of these. None of the manufacturers list my bike in their compatibility documents (ABE).
Magura states compatibility between their HC1 model and K75, but not K100. However, their HC3 model is compatible with K100. (HC3 isn't an option because of it's price (about USD 600-800)).

Then, I found something interesting, a German webshop selling the Magura HC1 as an K1/K100 compatible option: https://www.kettenmax.de/detail/index/sArticle/53298/sCategory/0
(Even though the manufacturer doesn't.)

Of course I could just trust Kettenmax, but I want to understand the criteria and what makes a brake cylinder compatible/non-compatible.

Both HC1 and HC3 can be delivered with different piston diameters, but the Magura ABE document says that the 15mm piston variant is correct for the compatible HC3. Kettenmax says that the HC1 with 15mm piston is compatible as well. Who to trust?

Here is what Brembo say about their options (with respect to the piston diameter):

• 19RCS to be installed on twin disks bikes with axial or radial calipers but NOT on Floating Calipers (My comment: Check! the K100 has floating discs)
  
  
• 17RCS to be installed on twin disks bikes with axial or radial calipers whose pistons diameter is 32mm or lower, but NOT on Floating Calipers; for example Ducati Panigale or RSV4, Yamaha R6 etc etc. (My comment: The K100 caliper pistons are 36mm)
  
  

Hence, I would guess that the Brembo 19RCS would work, - BUT I AM NOT SURE.




Some specifications about the original front brake system on my bike:




Disc: 2 x 305mm floating
Caliper: 2 x Brembo with 4 cylinders each. Cylinder diameter is 36 mm.
Master cylinder piston diameter is 20mm (but maybe axial)




Another question is why the Brembo model require a 19mm piston (to be compatible) while Magura is fine with 15mm.




I don't know if any of you consider such things this deep, but brakes are the single most important components on a bike, - so I just want to understand and be sure :-)




I would be thankful for qualified points of view!




/QuijoteNO

Hi all,

I have had a dialogue with Magura in Germany, and the short story (about compatibility between Magura HC1/HC3 and K100RS-4V or any bike with 20mm piston in the front brake master cylinder) is:

- I got a new ABE document from Magura (which is not yet on their web site) stating that their master cylinders (HC1/HC3) with 15mm piston are compatible with the K100RS-4V.
- Then I challenged them, and asked them to explain why they would recommend this product, HC1-15mm. (I thought maybe it could have something to do with a special Magura technology or something.)
- The case was escalated internally within Magura.
- Magura wrote me back, and thought my master cylinder piston was 13mm. I told them it was 20mm (standard).
- Their last mail gave me an apology, and they acknowledged that they were mistaken, and they would not recommend any of their master cylinder variants, - neither the 15mm nor the 18mm.

SO, I switched my focus to the Brembo 19RCS. Brembo is not that specific regarding compatibility with a certain make/model, but since they recommend their 19mm model to any bike with dual, floating discs and 4 caliper pistons larger than 32mm, AND the difference between the stock piston and their model is only 1mm, I chose to go with it.

---

So, next will be to resolve the case with a new, custom (gas) twist grip and maintain the function with the throttle level sensor. (Since the original twist grip and brake cylinder are built together.)

Thanks for the update - useful information.

Well done getting the resolution. The mathematics is very simple , so  Magura’s first response was a concern, cheers.

I'm wondering if for a non-4v BMW k100 I could use the Brembo PS16. I'm new to building/modifying the bike, but its front brake mc has the number 13 engraved, so my thinking was that a 16mm is good enough, wdyt?