My Avanti II 1981 model had the Kelsey-Hayes Proportioning valve on it and orignal Bendix Calipers when I broke the car down for frame off resto. Because the K-H unit is a bit of a rarity and hard to replace parts for, I decided to go with a brass GM PV2 proportioning valve . This valve is specifically for Disk & Drum combination and is a bolt-up replacement for the K-H valve. From what I have researched, a proportioning/combination valve is there for the purpose of allowing the rear brake drums to apply just ahead of the front Discs- that's all. This is to prevent the front brakes propping, and causing the rear end to 'come around'. For this reason, I wouldn't fob off the idea of using a combination/ proportioning valve. I wonder whether you might have a an issue with the right rear brake applying with more gusto- causing a right hand veer. May I suggest checking the orientation of the Shoes, adjustment of the shoes, the condition of the rear Flexible brake line (they are often blocked- like mine were), the condition of the brake lines along the rear Axle (I found mine crushed !) and condition of the rear brake cylinders. After that, maybe look at utilising a propvalve?