The 77 and 78 750F models (F2s&F3s) come stock with comstar wheels, dual discs up front and a disc rear. I am not sure why you are focused on the rear wheel as the vast majority of your braking power should be up front. Regardless, if you want to install comstar wheels on your auto and to significantly upgrade your brakes, your best bet would be to get a f2/f3 front end complete with forks, calipers, rotors, wheel and master cylinder. The forks would slide into your stock triple trees. On the back for a disc brake, you would need the F2/F3 swingarm, master cylinder, rear wheel, rotor, and proabably the brake pedal. You could also go for a comstar drum brake wheel on the back by using a 78 autos rear wheel. . . .By the way, your spoke wheels (assuming they are stock) are pretty nice pieces as they have lightweight aluminum DID rims. You could probably sell them to help offset the costs of the F2/F3 parts. . . .IMHO Comstars significantly detract from classic good looks of our bikes and swapping to them is a move in the wrong direction. They are also not all of a tubeless design (I am pretty sure the F2/F3 wheels are not tubeless).
