Greetings,

My bike is a 1991 (European spec) K100RS, and I did a full GSX-r front end swap, with the help of cognito moto.

I "assumed" that since the bike originally came with 4-pot brembos, that the GSX-r 4 pot Nissins would work just fine with the OEM BMW master cyl.

The front brake was spongy when I bought the bike, and the previous owner said it just needed to be bled.  He could have been lying, because I had to drill out one of the screws on the reservoir, so he likely never took the cap off. I never even attempted to try to correct it, because I knew everything was coming off.  I don't know if the MC is any good.

I just did the full install/bleed on the brakes.... no leaks whatsoever, but the brake handle is VERY spongy, and provides "a little" resistance, but only at the few mm from being fully compressed against the accelerator.

Nevertheless.... I have 2 questions:

1.  Is the OEM MC on my bike sufficient??  And all I need is to rebuild the MC???

Or....

2.  Do I need a completely aftermarket solution??

Thanks in advance!

What's the piston diameter in the GSX-R m/c? If it's 15mm or 16mm (as I suspect it probably is) then yes, the brakes will be spongey as the P08s fitted to the K100 use a 13mm piston. Also, are you sure the GSX-R front end has Bembos on it? Suzuki usually use Nissin calipers - I've never heard of them using Brembos.

Maybe they are Nissin.  Not sure.  Not sure what MC the gsxr came with either.  But, I don't want to do a "conversion" to all the gsxr controls.  I'd prefer to keep the BMW controls.

Actually, since I posted this, I found a lot of information that says the MC on my BMW is 20mm.... not 13.  Perhaps I am wrong?  Maybe someone can verify that??

Based on what I've read.. my MC is perfectly sufficient, it just needs a rebuild.

How do I tell?

Is there a visual difference between the 13mm MCs, and the 20mm MCs?

I haven't torn the MC apart, but I know the reservoir cover looked EXACTLY like the one in this ebay ad.  The ad says its for an 89-92 k100rs 4v... which is my bike.  And it also says its 20mm.

I don't want to buy it until I get confirmation from someone though.

https://www.ebay.com/itm/Magura-OE-Quality-Front-Brake-Master-Cylinder-Repair-Kit-BMW-K-100-RS-4v-/152691085175?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=707-127634-2357-0

ninefourteener wrote:The front brake was spongy when I bought the bike, and the previous owner said it just needed to be bled.  He could have been lying, because I had to drill out one of the screws on the reservoir, so he likely never took the cap off.

If he never took the cap off, he wasn't lying about the brake's needing to be bled.    Does the moto still have its ABS components? If so, did you bleed the ABS modulators?

If you are certain the moto is a 4V model then nominally, the eBay offering is the right kit. 13mm master cylinders have translucent fluid containers to allow fluid level inspection; the 20mm models have sight glasses for fluid level inspection.

ninefourteener wrote:
Actually, since I posted this, I found a lot of information that says the MC on my BMW is 20mm.... not 13.  Perhaps I am wrong?  Maybe someone can verify that??

Is there a visual difference between the 13mm MCs, and the 20mm MCs?

The 20mm front MC was standard on K1100 with dual 4-pot brembo callipers.  The dual two-pot brembos on the K100 used a 13mm.  Bowden cable on a 13mm is driven by a cam and chain, whereas the bowden cable on a 20mm is direct drive.  There's a cover over the cam/chain on a 13mm front MC.

13mm


20mm

ninefourteener wrote:I "assumed" that since the bike originally came with 2-pot brembos, that the GSX-r 2 pot brembos would work just fine with the OEM BMW master cyl.

If front brake master cylinders, front brake calipers and front fork assemblies are the topic here, a nomenclature problem seems to have arisen, ninefourteener. 2-pot refers to the number of pistons in a brake caliper. When a one-piston outline is seen on the outward facing shell of a caliper, it indicates it is a 2-pot brake caliper because there is also a piston on the other side. When two outlines are seen on the shell, it's a four-pot caliper. The OEM front brake calipers on both a 1991 K100RS and a '92–'95 GSXR are four-pot. As Dai indicated, the manufacturers can differ.

You're correct Laitch..... 4 pot.... not 2 pot.  Derp.  Perhaps my brain was tired from re-wiring my motogadget 7 times (another thread).

And yes..... I have the 20mm master cyl.  I just ordered a rebuild kit.

Thanks for your help everyone.

I'm also going to fix all my errors.... just in case anyone else reads it