Hey:
I don't have a 550, but a 650.
My 1980 only had one caliper, and the forks only had brake bosses on the left side. I inquired about this on the old greenspun site and so maybe some of the info will help you....
I bought a set of forks from a 1982 650 which has brake bosses on both sides -- if you have mounting bosses only on the left perhaps you'll need to find a set of fork legs from a different bike... Mine cost me about $50.
As far as hooking them up....
I had to buy calipers from the newer model year -- this was easy and I now have two sets of calipers, hoses, and master cyls -- I'll pick the best two of each and go with it. They were cheap, about $45 total. But, pretty much everything has to be rebuilt.
There is "T" hardware to split the hydraulics off to the second brake.
The biggest judgement here is whether to try and use your same master cyl or get one designed for dual discs. A single disc master cyl doesn't move the same amount of fluid volume that a dual disc master cyl does. So although it may work, you'll have to pull the lever much farther with two brakes than with one.
I like a firm lever, and so this won't work for me. Some said they didn't mind it but I'm going to get this dual master cyl rebuilt with a new reservoir and do it right.
After all, you're after more stopping power, yes?
Ride On,
Zeke
