1st things first.
If your brakes aren't performing like they should, your brake system needs attention.
1st, make sure that you have the correct "matched" parts. A master cylinder that is too big (I believe the stock single disc 750 K model is 1/2 bore,correct me if I'm wrong- I haven't putzed with old brakes in a very long time) like a 5/8 or bigger(single disc), you will have a "wooden" feel to your brakes (very little travel in the lever with hard feel when you are actually braking). If your dual disc-ing with a GL 1000 master cylinder (11/16 bore), get rid of it and get a 5/8 bore instead. Even the Goldwing people complain about the wooden fee that the stock 11/16 master cylinder has and there is a world of difference when going to a 5/8 bore (dual disc only).
If your parts (caliper, master cylinder) are all correct and your brakes still suck, rebuild them or at least start with a good flush of the brake fluid that is in there. Old brake fluid collects moisture and if your bike still has the same fluid in it that was in there when you bought the bike, replace it. Replacing the fluid didnt do it? Rebuild/replace the parts to get the brakes back into working order. Just slapping on another old used part may not fix your problem so rebuilding your existing "rebuildable" parts is often better to do.
Improved braking
Replace the old rubber hoses with braided brake lines. New braided brake lines give a better feel and a "more direct feel" (?) because the braided brake lines don't swell with pressure like the stock rubber lines do.
Dual discs-ing
When you get the 2nd caliper,disc,etc and get it all sorted out, put your braided brake lines on and try it out with your stock (single disc) master cylinder. The "stock" dual discs that I have done in the past worked great with the single disc master and braided brake lines. There is slightly more travel in the lever, but it has worked for me in the past and the lever did not run out of travel(hit the grip). Since you already have the single disc master, try it first (sensibly).YMMV
If you feel the GL1000 master cylinder(11/16) would be a better choice, get a 5/8 inch instead.
Newer master cylinders
If you feel the need to get a newer sport bike style master cylinder, get one of the proper bore size WITH an adjustable lever. With the adjustable lever, you can adjust the braking feel (where pulling the lever starts to make the bike slow) to be at a comfortable finger reach. Having the braking "action" where it is uncomfortable on your hand/fingers isn't going to inspire any braking confidence. When going to the sport bike master cylinder, you can "sometimes" go to a one size smaller bore and adjust the lever out to get more travel and a more precise feel (the braking action is spread over a longer lever throw).
Still isn't stopping anything like your CBR?
The stock single/stock dual disc used technology that is 30-40 yrs old now. Brake technology has come along way since these bikes were new. Keep it in perspective. These old bikes can be improved some but the technology has come along way since these bike were "state of the art" in the 1970's. It ain't going to stop as good as your '03 YZF/R1/CBR/TLR.
