Hy 1pete1, it doesn't actually mater where in the braking system plumbing you install the proportioning valve, what matters in your case is how you augment the proportioning valve. In a braking system with front disc/rear drum most of the braking effort is in the front brake system, disc brakes require more pressure, and if that increased pressure was allowed to be sent to the rear drum brakes those brakes would lock up causing a dangerous skid situation. To be honest with you I am not sure how your brakes will perform, I think that maybe an adjustable proportioning valve installed in the front brake circuit and adjusted to give the best braking effort without constantly locking up the front wheels may be the best way to set up your unique system. I am sure others will be along with suggestions as well, good luck with getting it to work appropriately.
