1-rotor to 2-rotor master cyclinder

[teresaqm]

If I take my R65 from one to two front rotors do I have to change the master cylinder?  If yes, what's the best modern on to put in?

[rl900]

The stock single disc M/C is 13mm, the dual disc uses either a 15mm or 16mm depending on year.  The 13mm will give too low of a mechanical advantage and feel wooden.  The 15 mm will give better feel and power.

If you don't go with the BMW (Magura) then you will need to replace the throttle and switchgear as well.  The BMW cylinder is about $215 and a 15 or 16mm Brembo or Magura will be $85-$130.  Alternatively you can get a used Japanese master (Nissin or Tokico) in 15 and 16 mm.  
Ron L

[Crossrodes]

 I have 2 rotors on my '79.  I'm not sure which size master Cylinder I have.  Is the 13/15/16mm size the measurement of the outside diameter of the cylinder?

[Ed Miller]

The number is cast into the MC body, somewhere.  At least it was on mine.

[drewboid]

I added the second disk and kept the 13mm original master cylinder. I can almost get the lever to bottom out against the grip with a really hard squeeze but my fiance cannot. For her it is ideal; betterstopping with less force required.  Replace the hoses with SS - the rubber expands too much under pressure.

My advice (free and worth every cent) try it - if you don't like the feel then go to the larger master cylinder.  BTW - the MC should have the piston diameter cast into the bottom.   YMMV.

[nhmaf]

Yes, some prefer to have the longer travel, more gradual pressure build up with the smaller diameter master cylinder with the dual brakes.
I have the large one (16) on my 82 LS, and sometimes it seems too sensitive   I cannot squeeze it more than about 1/2 to the bar with less than
4 fingers, and cannot squeeze it enough for it to bottom out.   Coming to a very hard stop usually requires only a couple finger's worth of pressure
and lever travel of about 20% full range.   It wasn't always quite so powerful, but I switched to braided stainless steel brake lines that seem to have absolutely "0" compliance so all the hydraulic pressure is going right to the pistons and not swelling the hoses.

I've done 1 unintentional "stoppie" with it - but didn't drop her !

[steve hawkins]

There always the oddball!  Here I am

I went from dual disk to single disk.....Just to be different.

Are you touring?  Are you loading the bike up?  Is your current setup working properly?

If not, there are plenty of ways to improve the single disk without fitting another - Think of all that weight man  Right where you do not need it - i.e. its unsprung weight!

Think of the cost?  You will have two of everything to service - well disks and calipers and pads!

Things you can do with a single disk set-up:

Fit braided hoses, if you have not done so already.
Fit a modern full floating disc and recomended pads (but only one )
Service your brake system to ensure maximum efficiency.

How much is a caliper/disc and master cyclinder these days? oh someone has already answered that one above.

An alternative view from an alternative guy.  However it has to be said that my bike is used for fun and commuting to work and if I can remove some weight from it, then I can eat more pies.....erm, sorry, go a little quicker!

Steve H