If you had better brakes, you might have been able to avoid the collision, or at least lessened the impact. The 12mm MC is probably the easiest upgrade for a 400; since the forks only have mounts on one leg, you can't do a double disc like on the 550 and 750. In order to mount a better caliper, you need to change the fork legs to ones that have the mounting lugs on the back of the legs, like the 750 F2/F3 and GL, which is why I mentioned the CM400. Tim, would the CB400 wheel have worked with the CM front end? Are the fork tubes the same diameter as the CB; if so, they should slide up into the CB triples.
The AP and Grimeca calipers have two opposing pistons, and have no hydraulic "leverage" advantage over a single piston of the same size since they cancel each other out; the only reason to use them is to comply with "vintage racing" rules.
The two piston Honda calipers I referred to have both pistons on the same side, which doubles the piston area, so a 32mm caliper has as much area as a single 45+mm piston. There may be a factory bracket that would allow the two piston caliper to bolt up the CM fork leg, just like on the F2/F3 and GL.
I swapped the forks on my Seeley to the late F parts just so I could use the two piston calipers: I had noticed that my '82 FT 500 Ascot had an excellent front brake, with a rotor the same size as a stock 750, a 14mm MC, and a single 30mm two piston caliper, which is just shy of the area of a F0/F1 K7/K8 42.8mm caliper. I did some testing and found the two piston caliper performed much better than a K7 caliper, then adapted a 320mm Ducati rotor with a 32mm caliper and HH compound pads. The end result is braking on a par with or better than a dual disc setup, and less weight than the stock 750 single disc setup.
