Alternate Master Cylinder to Replace Round ATE

[Carott]

I have tried to search for an answer with no success.
My 1980 R45/N has the round ATE master cylinder and the reservoir is in terrible condition and I suspect the piston and cylinder are not much better.
I would like to find an alternate master cylinder that is compatible with the existing perch.
Does such a beast exist or should I be changing over to a perch that works with the Brembo master cylinders?
Thanks in advance,
Brett

[Tony Smith]

The later "square" master cylinder fits the existing perch.

You may also have to change the brake lever.

Having said that, someone recently reported on the Facebook Airheads page that a round reservoir that fits can be purchased direct from Brembo, the poster claimed to have done so and fitted it to his bike. BUT, he has not responded to follow up questions.

[Carott]

Tony,
Thanks for the quick reply.
It was a real Homer Simpson "DOH" moment for me.
I had picked up a used 15mm rectangular master cylinder and bought a rebuild kit for it last summer.
Somehow when I compared it to the round ATE I convinced myself that it was not the same and that I needed to change the perch.
After reading your reply I pulled off the ATE this afternoon and the Brembo slid right in.
DOH [smiley=embarassed.gif]
So now I will have a rebuilt M/C, new SS braided line to the distribution piece and a new Brembo F08 caliper from MG Cycle.
If this doesn't make it stop on a dime, I'm not sure what else to do.
Thanks again,

Brett

[davidpdx]

A 15mm master cylinder may be too large if you have a single disc. I just removed one from my bike and replaced it with a 12mm and the improvement in braking was remarkable. See stainless steel brake line thread under Chit Chat for the whole story. 

[Tony Smith]

With a 15mm master cylinder and a single F08 you are going to need to develop  a grip that can crush walnuts.

Assuming you have a standard 36mm F08 (although if you have a 38mm one that will mildly improve matters, you have a hydraulic ratio of either  0.208 (36mm) or 0.197 (38mm).
 
By comparison, a standard single F08 (or ATE) will have a 12mm m/c and a ratio of 0.166

A standard twin disc machine will have a 13mm m/c and two 36mm F08s (or ATEs) and a ratio of 0.090

And my own machine with a 12mm m/c and twin 38mm ex-K100 F)*s is 0.078

First things first -the above maths is approximate as I have simply used diameters and not swept area which would be inherently more accurate - but dimple diameter ratio is good enough to show how very different things can be.

Now a lot of this is down to preference - at opposite ends of the scale a 15mm m/c with a single disc is going to be "wooden" and be a very hard pull to get brakes. My bike is at the other end of the scale to an absurd degree - in fact I have my fellow club members queuing up to ride it and experience the "power' of the 860cc engine and I will not let them ride it until I have briefed them on the brakes and got them to do a few low speed stops - the reason is that I can lock the front wheel with my little finger alone. To be blunt it is dangerous and as soon as I can source a cheapie I'll fit a 13 or 14mm m/c to tame the brakes somwhat.

So, if your brakes are too bloody hard, and mine are too bloody powerful I reckon the "sweet spot" for a single disc system is 12~13mm and he sweet spot for a twin disc is 13~14mm.

If your 15mm m/c is in good condition I would suggest that you buy another caliper and disc and go twin discs, the costs will work out the same as a new 12~13mm m/c

[Carott]

Thanks to Tony and Davidpdx for pointing out the importance of matching the master cylinder size to the calipers.
The replacement master cylinder I have is actually a 12mm unit but my brain decided that I should write 15mm in my post.
I am beginning to think that I may not be competent enough to be allowed to work on a critical safety system like the brakes without adult supervision.

Brett

[Barry]

13mm was the standard size for a single disk 78-80 models.  It changed tp 12mm in 81 even though the caliper pistons were the same size.  So a small improvement, perhaps BMW realised they had been too cautious. If I ever need to replace mine I'll definitely go with something smaller than 13mm even if it's only the small step to 12mm.

[davidpdx]

If you got the new pads and lines on the swapping of the master cylinder and brake bleed is easy. You will love the results and who better to work on the brakes than the one who is depending on them.

[Carott]

The changeover is complete.
Fantastic two finger braking and no leaks.
Now on to the next part of the project, reinforcing the used dash cover I bought so that it doesn't tear like the old one. Layers of Goop and fibreglass window screen should do the trick.
Thanks for all the help.
Brett