Disc/drum and disc/disc are completely different situations. The factory never made a disc/disc system, so you are on your own. Modern disc/disc systems do not use proportioning valves — they control the amount of braking front to rear by balancing the overall stopping power — the size of the rotors and the number of pistons in the calipers. The factory proportioning valves were designed for disc/drum systems, and they do not control the total amount of braking force at the rear brakes — what they do is modulate the rate at which the braking force increases due to brake pedal pressure, to compensate for the different response characteristics of drum brakes versus disc brakes. Disc brakes respond linearly; drum brakes do not — as more shoe surface contacts the drum, it pulls itself tighter, creating an increasing, non-linear braking effect. This is what the proportioning valve compensates for, by tapering off the rate of line pressure increase.
What this means for your unique disc/disc system: you will have to test it on the road to evaluate its front/rear balance. Since the rear probably has smaller rotors and calipers (you didn't describe them), it may work well as is. If the rears lock up prematurely, than an aftermarket adjustable proportioning valve (such as Summit Racing's) should be installed in the rear brake line. This will let you adjust the line pressure to modulate the rear brake response.
Tip: you do not need to cut and flare brake lines to install the PV. Just buy a short pre-made length of brake line at your local auto supply, and bend it into a short loop. Then you can undo the rear brake hard line from the combo block and route that to the PV, then insert the loop between the PV and the combo block. This locates the PV down low near the combo block — a lot of people think they have to locate it up high in the engine compartment where they can easily reach the adjustment knob, but this is unnecessary — you will only need to adjust it a few times over the first few rides. After that it will not need further adjustment.
To 66jim: The first test is whether the rear brakes lock up in a panic stop on dry pavement. Once that is adjusted out, then a wet test may make sense, if a safe location can be found.
