Adding a second brake caliper to a '79

[Barry]

Barry, I respectfully disagree.
I bought my bike new with ATE calipers (dual set-up). It is an 1982 model and had the square bowl master cylinder. It was 15 mm dia. The square bowl came with the 1981 model, no matter what caliper the bike had.

BMW were known for using up the parts bin. Interesting that that you have a 15mm master cylinder even though ATE's were fitted Still it's better than a 16mm that earlier bikes had.  

If you guys with twin disks think a 15mm is wooden spare a thought for those with a 13mm and a single disk which is a much worse hydraulic ratio. There is almost no handlebar travel at all.  And that's where the Hammer65 the OP is with a 79. You will have gathered by now that's it's not just about increasing disc/pad area but about improving the hydraulic ratio that BMW were so conservative with.  I thought it was a step too far but if you can get away with keeping the 13mm after adding a 2nd disk. That should give a vast improvement in the hydraulic ratio and feel.

[mrclubike]

I am with  Barry
I would try the 13mm MC with the dual disc before  buying a 15mm
With SS brake lines it may be very nice
I know my 13 mm MC with a single 38mm F08 and a SS brake line
I have very little lever movement
But it stops great.
When I bought the bike the PO had put a 16mm MC on it with the old single 36mm ATE. I didn't ride it like that but I bet it  needed a lot of lever pressure to get it to stop  

[lucky]

I am with  Barry
I would try the 13mm MC with the dual disc before  buying a 15mm
With SS brake lines it may be very nice
I know my 13 mm MC with a single 38mm F08 and a SS brake line
I have very little lever movement
But it stops great.
When I bought the bike the PO had put a 16mm MC on it with the old single 36mm ATE. I didn't ride it like that but I bet it  needed a lot of lever pressure to get it to stop  

Which model year of 13mm MC did you source from?

I have very little play with stock 15mm MC and single Brembo up front.

I do have the parts for dual set up, but I am still debating do it or not.

[wilcom]

Lucky.......... I'd set it up with the 15mm MC and see how you like it. My dual ATE's with a 15 gave me 2 finger front brakes