The cbr1000rr 08 up works fine on the vw disc brakes althouth you do have to modify the gl1800 right control switch and throttle some. the crb brake hande is a little short but works they want 99 bucks for a longer handle. maybe someday
I agree you need a residual valve if you are running out of pedal travel, sound like you are losing to much line pressure at the brake.
I installed a residual valve on my GL1500 trike, for the rear disks, it works great.
Think of it as a check valve, that holds most of the pedal/line pressure, so when you go to apply the brakes, you dont have to press the brake pedal so far, to actuate the brakes.
A larger brake master piston would apply if you needed more braking power, larger the piston the more power but the harder the pedal becomes to press.
i have two 10# rv in the front one to each wheel