Brake Cable Mystery 75/6

[Planecrazy13]

When we got this bike it was missing the lock nut part of the adjuster (circled in orange). I recently rebuilt entire brake sytems to include xable and new adjusters. Then I discovered why that part was missing. The cable is not long enough with it it on. If you remove ot there is just enough cable length to make it work.

Anyone ever come across this before? I have verified all parts at least three times, including the handle itself.

[schrader7032]

What does the other end of the cable look like on the under tank master cylinder? There is a specific adjustment that must be made at the back of the cylinder where the cable connects. Snowbum discusses this about 1/2 down the page...just above a few pictures of some tools:

https://bmwmotorcycletech.info/brakes.htm

That u-shaped tool is used to ensure that the master cylinder piston is positioned correctly which then affects the cable attachment. Be sure that's properly adjusted.

If after that, you have no adjustment at the lever as you indicate, likely you need to get a new cable. From RealOEM it would seem that a '75 R75/6 brake cable is 653mm long. But there are other versions of this cable that was used after that, and they are 659 or 679mm long. The longer one is probably used for the high bars used on the /7.

[Planecrazy13]

Thanks. There is zero room for adjustment. I have tried two new OEM cables and have same issue. Getting a longer one is the solution to being able to use the jam nut but it still does not explain why there is no free play with original spec cable. Hmmmm.

[schrader7032]

And the connection at the back of the master cylinder is per spec?

[Planecrazy13]

Yes. By removing that nut it falls exactly into spec. Adding the 659mm cable would add just enough cable to put the nut back on and leave a 1/16 in (maybe).

[srankin]

Did the previous owner do a handle bar change, adding taller bars? I see you have checked part numbers, are the bars stock or aftermarket?

Forgive me, it has been a while since I looked under the tank of my 78RS to see how the master cylinder is mounted to the frame. Is it possible to reposition it to give you more slack on the cable?

Silly questions, and answers but if you are the second owner, you may find previous owner bug has infected the bike. Cheers, St.

[ECJ]

I also suggest checking the position of the MC under the tank (I believe it's secured with a single band clamp, and sort of registers on a 'boss' on the frame) as well as the possibility of different bars having been installed. These seem like the most likely possibilities....

Good luck,

ECJ

[Planecrazy13]

Everything positioned properly and even a little slack turned both ways. But, the way the cable is made it would not allow use if positioning wrong(too far away).

Still a mystery.

[Carott]

PlaneC,

I asked myself the same question about the lock nut when I was working on my 1974 R90/6. I went as far as to buy a new nut and install it. Didn't work. Then I looked at my 1979 R1000RT which I have owned since new and it didn't have the nut either.

The brake cable for the under tank master cylinder was never intended to be adjusted at the handlebar and that is why your bike does not have a lock nut there. The microfiche is incorrect. If you check you will see that the handlebar perch is not threaded where the cable "adjuster" goes into it. There is nothing for the lock nut to "lock" against. Compare that to the clutch perch and you will see the difference.

I believe that the reason BMW set it up this way was because owners who were used to having cable operated drum brakes would regularly adjust the cable at the handlebar to accommodate for wear and cable stretch. If an owner with the under tank master cylinder adjusted the cable at the handlebar they would be in danger of preloading the piston in the master cylinder to the point where it was past the internal refill/relief ports. This would possibly prevent the brakes from operating correctly.

Brett