OK, I assume someone is always interesting in a technical discussion and after some further searching I found it is most likely a Nisin master cylinder at the rear, and the cylinder in this video (

) is probably an older version, but the rod and the fork connecting to the brake lever is identical to the one on my '22GS. As the rod have a spherical steel end on the inner end, it is obvious that rotation of the rod does not affect any items inside. Also the flat area and the upper hex is identical.
This only confirms most of the above discussion, rotation of the rod is only causing the fork moving up and down on the rod and nothing else.
As there are almost nothing to adjust on, and I believe BMW want the brake lever to pull the rod out when not operated, I think they says it shall not be adjusted because there is simply not enough freeplay to make any difference, my best guess.

 

Thanks, I'll keep that in mind. I usually replaces the fluid every year, the BMW fluid is half the price of the Yamaha I used on my FJR and I enjoy doing maintenance work as well. As the bike is '22, I changed the fluid for the first time this year and got rid of a lot of air by mounting a check valve and bleed fast and a lot, both front and rear became a lot more firm and feels very good, hopfully it remain like this for a year.

 

That brake lever will change as the fluid gets old. With the BMW brake fluid in mine I had to press further and further for it to work as the miles went by. People on the adventure forum said the catalytic converter is a bit too close to the master cylinder. The fix was to replace the BMW fluid with one with a higher boiling point. Motul 660 DOT4 worked very well for me and the pedal no longer goes very far down before grabbing.

 

Thanks for the photos. The lower part the fork is definitely different, but it make sense that the upper part is identical and I agree the upper hex is not a nut but a feature of the rod. If so, the only adjustable is the distance from the upper part to the centre of the brake lever pin. The rod lenght out of the master cylinder will remain constant. So to adjust the fork, there are two options, keep the fork steady and rotate the rod, or to keep the rod steady and rotate the fork (brake pedal disconnected). So I assume as long as the pedal have enough freeplay do not touch the frame all shall be good.

 

This is the stud from my R1200GS. Your R1250GS may be different. The long cone shaped piece simply slips up into the master cylinder boot. Adjusting the length of the studs engagement simply varies the amount of brake lever travel.

 

Not sure, the fork as I call it can be rotated without rotating the rod if the pedal is removed and the lower nut loosened, so no need for rotating the rod for adjusting the pedal position. Regarding the upper hex, which I think is a nut, you can see the threads are not stopping outside but goes into what I believe is a nut, but that really doesn't matter as long as rotating the nut/hex/rod is not messing up anything inside the master cylinder. I have full control on the brake pedal position, I am only not sure what is inside the cylinder unit.

 

The upper "nut" is not a nut but a feature of the stud allowing you to turn the stud further into or out of the brake lever to adjust the amount of lever travel before the brakes are applied. The only nut in the assembly is the jam nut used to lock the stud in place once the adjustment has been made.

 

Agree, but by turning the upper nut the rod turns as well (not holding the rod back), and the marked distance does not seem to change. I see there is a flat area of the bottom end of the rod, but I have not used that, hence I believe I have not adjusted the rod length as both the upper nut and the rod has been rotated. I might be wrong of course.

 

The two dimensions you've asked to be measured only gives you the length of the adjustment stud on the master cylinder. All should be the same length. The brake lever position on that stud can be adjusted up and down the length of stud but the length of the stud will not change.

 

Ok, the brake is fine, I see what you are saying about adjustment. My does have a shorter value than bosnjo did measure, but what I see is that the upper nut is quite tight on the rod, so turning that nut turns the rod as well and do not change the distance. To adjust this nut I believe you have to hold back on the flat area of the rod at the bottom, which I have not done, so I start think that I have not done any adjustment at all except on the position of the fork claw, but the gap between the pedal and the frame is fine and as before. Of course I do not know how thing are inside the master cylinder, if there are another nut inside that does not rotate and is then causing some internal adjustments when the rod is rotated, but I wouldn't believe so, it would be to easy mess up by rotating the fork claw when the pedal is removed due to maintenance.

 

The number will Varley slightly per bike. These adjustments should not be used to raise/lower the pedal height. If you adjust it one way you'll have no rear brake as the plunger will not be able to travel far enough to actuate. If you adjust it the other way you can make it so even the slightest pressure on the brake pedal will give 100% braking force.

You can adjust the plunger on the bike to actuate properly, it's not a measurement thing. Adjust apply the brake and feel when it engages. If too fast/soon engagent move the plunger away from he MC. and recheck. If it does not engage fast enough move the plunger towards the MC and recheck.

A little adjustment gosa long way. We are not talking multiple turns sometimes its a fraction of a turn to get it adjusted properly.

 

 

I've just measured it on my 2021 GS.
The shorter distance is 19,5mm and the longer one is 40mm

 

Hi, perfect, thanks a lot, will check against that. If anyone else can measure as well, that would be great. Good to have at least two values to see if they are equal.

 

Hi, waking up an old thread as I will gently ask someone that have a newer 1250 to take two dimensions. I know now that I most likely shouldnt adjust the upper nut on the rear brake master cylinder so not starting that discussion, but I was a bit quick to read the manual. I have reset as good as I remember to oe position, but hard to remember exactly how much I turned the nut. I have no issues with the brake, have test rode the bike for 30 km, and all fine. Anyhow it would be nice to check the position against an unadjusted nut, so if I can gently ask someone to measure the two dimensions marked on the attached photo, that would perfect. Not sure they are all the same or if they are individually calibrated, but as it is all machined items, it can't be much off. If so, it would be the first time I ever heard about a calibrated master cylinder. I am not to concerned, just want to do a simple check.

Thanks
/rogerf

 

People have complained about the back brake for a long time. Don't adjust it. I had to press mine too far before it engaged. Bleeding the back brake with Motul 660 fixed mine. https://www.amazon.com/dp/B005UGKKW...ypp-rep_ypp_rep_k0_1_21&amp&crid=N88GPKIIRQGE&amp&sprefix=motul+brake+fluid+660

 

Those are just some of them. If you go to AliExpress you have some very good quality copies for a fraction of the price. For example, if you write "r1200gs brake pedal" or "r1250gs brake pedal" in the search box, you'll get some items for as low as 5$.

 

Thank you bosnjo! Those never popped up in my searches.

 

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