Aha, I’ve searched several recyclers, and I found a listing for a proportion valve for a 78 truck locally. I wonder why it’s not mentioned in parts catalogs? Maybe they don’t need replacement much? On further research, I did find a catalog that listed a discontinued prop valve for the RN30 trucks. It says ‘load sensor’, which might be hard to implement on this truck that wasn’t designed for it, but I’m thinking of giving this recycled one a try, to see if it makes this perform differently than my jegs valve.