These instructions are for a Honda CB 500 or 550 four, they will work for 750 K3 onwards if you get 750 parts where 500/550 are mentioned and earlier ones if fitted with the later fork slider assemblies
Hi easiest way is to collect the following bits:
(1) Goldwing (GL 1000 or 1100) or other Honda twin disc master cylinder complete
(2) 500/550 disc SH
(3) 500/550 caliper with mounting brkt and lower hose c/w banjo bolt and washers
(4) Speedo drive 44641-300-000 (now obsolete but Western hills Honda Cincinnati did have a couple in stock--DO NOT accept the modified number it wont work, there is a photo of the required plate on page two from which you should see how to modify the drive you have)
(5) Two new seals for caliper, part number 45108-300-003, and preferably two new sets disc pads
(6) Six 8 x 110 HT bolts with nylock nuts.
Strip out, clean (especially the seal grooves) and rebuild both calipers with new seals, remove front brake light switch from block bolted to bottom yoke (lower triple tree?) and connect the second lower hose to the hole uncovered (same thread on banjo bolt), Replace master cylinder with twin disc type (if you got a "late model" there will be a brake light switch operated by lever blade. If so, connect this to wires from removed switch--if not rely on rear brake light switch).
On front wheel remove front wheel spindle and Speedo drive gearbox, remove and discard six disc rotor holding bolts, chrome cover and Speedo drive plate.
Fit New? Disc with longer bolts and nyloc nuts. Fit replacement Speedo drive plate.
(This may necessitate cutting part of the aluminium disc center out to form two rectangular slots). Refit the Speedo drive gearbox and wheel spindle.
Bolt wheel into forks and bleed brakes. You now have twin discs using all standard parts if it still wont slow down enough you are driving too fast.
If you want the picture that goes with this you will have to PM me with your email address but you won't get an answer till after 19th as i am away from now