Duster proportioning valve help!

Hi guys. I have a 74 Duster and am in desperate need of a proportional valve for the brakes. I have disc on front and drum on rear and are factory. Any ideas what I could use and where to get one? I have looked around my area and have had no luck. I dont think I need an adjustable one, but at this point something is better than nothing. :dontknow: