I was never able to get satisfactory brake feel with the stock cable-operated master cylinder on either my R90/6 or my R100. Braided-steel brake lines are a big help, however, there is an even better alternative...
For $250 to 300, you can convert dual ATE brakes to use a bar-mounted master cylinder with braided brake lines. I completed this conversion for my '79 R100S in 1997, and as of 2005, all the parts are still easily available. Note that you can probably still get a kit from CC Products in San Jose, California that does a similar conversion, but it will cost more, and will not give you quite as good results, because the brake lines are longer and go through a T-adapter.
Here are the parts you need to do this yourself for 1975-79 R-bikes with single or dual ATE brakes (1974 support is unknown; someone told me that the 1974 R90S had some type of difference that prevented this specific conversion from working):
Magura 14, 15 or 16 mm master cylinder assembly, apparently Magura part number 225 348. The box also had Flanders part number 47828625, and a marking "286.25 lS/o.Gr". Kari Prager of California BMW says that you can also use the master cylinder from a 1985-on R-bike with dual throttle cables. If you do, look for one with a 14 mm cylinder; the 12mm cylinder won't work well with dual brakes. A 15 or 16mm cylinder will give a firmer feel to the brakes, but I am happy with the 14mm cylinder. If you are converting a /6 with a single front ATE brake, either the 12 or 14mm cylinder should work fine.
For 32 mm Bing carbs, BMW throttle cam #32721 457 038, or for 40 mm Bing carbs, BMW throttle cam #32721 458 092. This throttle cam is for the European GS models, and has a moderate ramp giving you medium throttle action. If you want a faster throttle, look for a cam (there are at least 8 different ones available) with a bigger ramp, a pivot hole that fits onto your new throttle assembly, and a throttle cable connector for dual cables. You might want to get the master cylinder assembly first, then go to your dealer get a throttle cam that fits the pivot.
Braided steel lines and new brake line grommets to fit. Now you have a choice: either use a double banjo bolt to directly connect the lines to the master cylinder, or use an adapter from CC Products. Here are some details.
CC Products sells a conversion kit with a T-block that mates a single brake line from the master cylinder to a pair of lines to the calipers. This block allows you to use the stock brake switch in the bike, and you can order it separately (and it should be in stock!) This adapter will also let you do the conversion on a bike with S-bars without changing the instrument cluster bracket (more details below). However, I set my bike up with dual lines direct from the master cylinder, using a double banjo bolt setup. This will give you the best brake feel, somewhat better than the T-block approach. If you use a double banjo setup, you need: custom braided steel lines, with the double banjo bolt to mount them to the master cylinder.
I got my lines, master cylinder and double-banjo bolt from Kosman Specialties in Windsor, California, for $125 in 1997. Kosman is happy to deliver parts via mail order. For S and RS bars, the lines need to be 30 and 31 inches long. Kosman does customer setups like this all the time, so they should be able to build the lines easily. Their web site is http://www.kosman.net, and their number is (707) 837-0127 (as of 2005). Remind them to grind 2 mm off the end of the banjo bolt so that it will screw all the way into the master cylinder assembly.
I used a pair of BMW #34 32 1 233 159 grommets on the fork legs for the new brake lines.
The conversion is pretty straightforward, and takes about 2 hours, with beer breaks, if you have all the parts:
Disconnect the fuel lines and remove the tank. Disconnect the lower end of the brake lines, and drain them carefully. Take the lid off the master cylinder, so it drains too.
Screw the brake cable adjuster in on the left handlebar, unhook the cable, then screw the adjuster all the way out to disconnect the cable.
Put the cap back on the master cylinder. Unplug the electrical connectors from the brake switch on the master cylinder, then carefully remove the old master cylinder and old brakes lines completely. Use rags rubber-banded over the ends of the brake lines to keep brake fluid from dripping out.
Remove the throttle cam cover on the right grip assembly, disconnect the throttle cables, disconnect the switch cluster, and remove the old right grip assembly.
Loosely mount the braided lines to the new master cylinder assembly with the double banjo bolt and with copper washers in between the brake line terminators, and mount the assembly on the handlebar. Position the lines to run between the forks in front of the upper triple clamp, and behind the lower triple clamp. Tighten the banjo bolt with the lines in place, once you are sure that they will clear everything as you turn the fork from lock to lock.
Install the throttle cam with grease, reconnect the throttle cables, install the throttle grip with the correct cam alignment and more grease, and screw the throttle cam cover on. Extra hands may be helpful :-)
Replace the brake line grommets, and attach the brake lines.
Install the new throttle switch, route the wire down behind the head tube, and plug the male spade connectors into the original brake switch female spade connectors. Zip-tie the wires in place, making sure they don't bind as you turn the fork from lock to lock. With the ignition on, make sure the switch is activating the rear brake light.
Fill the master cylinder and bleed the system. You will probably also need to re-synchronize the carbs.
It is amazing how much better the ATE brakes feel with a handlebar-mounted master cylinder and steel lines. With good soft pads, the front brakes work very well and are up to modern performance standards.
Kudos to Colleen Bray now at Bob's BMW in Maryland, and Bob Schenker in Oakland, CA, for helping me put this together. It was Bob's pioneer work with his 1981 R100S brakes that convinced me to do this.