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Major Softie wrote:Don't rush into getting a large MC. Early Airheads had MC - Caliper piston ratios that were far from optimum, and smaller MC pistons make the braking much better. Check here for specific numbers:

http://www.vintagebrake.com/mastercylinder.htm

Good advice and an excellent source to determine a vastly improved MC:caliper ratio.

Duane,

The battery charger has an indicator showing when the unit is fully charged. It does crank pretty good, but I don't feel it has enough reserve amperage to do the job. We are talking about a 10 year old battery, I believe, if it matches the last registration I found in the tool tray. Since it has been charging my next move is to go to Batteries + and do a load test.

As far as the MC, I was checking the parts fiche and it seems that 14mm was stock for the 90/6, but the 90/S used a 16mm till 9/75 then they went to a 17mm. Based on the replacement prices I have seen, I may not rush to change it yet.

14mm was stock on a single disc bike, the larger unit used for twin discs.

The major is correct, BMW cocked up the ratios between the master and slave cylinders resulting in very wooden brakes, I think that they were worried that people would lock the front wheels.

I use a 13mm handlebar MC from a r100r, bought new from Motobins, the ratio is perfect for the twin 38mm solve cylinder brakes on my R90/6, giving we much more feel and power on the front brakes, the brake lever comes back much further towards the twist grip but I do not run out of brakes even when provoked by a series of full on stops from 70mph. The brakes are still not as good as the brembos on my 82.

Yeah, the more I hear about these ratios, the more I am inclined to let them be for the present. When I converted my Connie from the stock calipers (looks like they are of the same single moveable puck type) to some dual 4 piston units the MC was left as stock. The brakes really work good, there is wonderful progressive braking and I can lock them up with a bit of effort. I really like the progressive feel as I do mountain riding daily and I like to drag them a bit going downhill into curves.
thanks for the link to the brake ratio site.

regards,

You may have to drill your 14mm MC to get 2 brake lines for the twin discs

chasbmw wrote:You may have to drill your 14mm MC to get 2 brake lines for the twin discs

Nonsense. This will be only one option –
One can also fit a a splitter at the mc or use a custom split line.

I'd bet that chasbmw's method (13mm handlebar MC from a r100r) works quite well !

I have a 11 mm m/c with twin ATEs and it works well, without any excessive lever travel.

Most Jap cars have the same thread as BMW brake fittings, so a splitter isnt hard to locate if you want one - they are usually still bolted to the front of the firewall.

Or you could follow the factory and use a single line to the RHS calliper, then run run a solid steel line from the bleed nipple up and over over to the LHS calliper.

I have seen a nice set up on a few Kawasaki K1s, with a fancy splitter fitted to the bottom if the lower triple, if you fancy a custom look.

jagarra wrote:Duane,

The battery charger has an indicator showing when the unit is fully charged. It does crank pretty good, but I don't feel it has enough reserve amperage to do the job. We are talking about a 10 year old battery, I believe, if it matches the last registration I found in the tool tray. Since it has been charging my next move is to go to Batteries + and do a load test.

As far as the MC, I was checking the parts fiche and it seems that 14mm was stock for the 90/6, but the 90/S used a 16mm till 9/75 then they went to a 17mm. Based on the replacement prices I have seen, I may not rush to change it yet.

Your battery is 10 years old? As in "ten" years? I would turn it in on warranty, as it should have failed 4 years ago. Something is wrong with it for putting out anything at all at this late stage. I would want it fixed to operate as it should.

Seriously, don't even mess with it, go get a new one. At old age you are fighting several unknown things inside that make it useless to bother with.

chasbmw wrote:You may have to drill your 14mm MC to get 2 brake lines for the twin discs

One could also just make a splitter down by the caliper...worked pretty well for mine.

After I pulled the battery, what a pain, had to loosen the bat box and remove the bolts holding the rear section on. I saw a date scribed on it with an 02 date. Ten years old alright. Took it to Batteries plus and we did a load test on it. It held 12v and 300 cranking amps for a few seconds, then dropped like a rock. Got a replacement, lead acid battery the same large size (that bearly fits between the frame) for a great price and put it on a trickle charger to get it up to full charge.
Made a little mistake when I left and didn't turn off one of the petcocks, found a nice puddle of gas under the bike, leaking from the right carb. Guess I have to check that out next.
Have yet to pull the tank and check out the fittings needed for the MC, if it's a banjo style they make nice stacking fittings for dual lines. Not a priority as getting it running to evaluate the condition is becoming a higher priority now.