I don't think the prop valve is the problem, especially since you only lock up one wheel. If it had been both wheels, then it could be suspect. One thing I can think of, besides the parking brake cable being too tight, is potentially a hung up equalizer bar. If it's hung up, it will hold the shoes closer to the drum that the other side. Adjusting it down generally won't help. You might want to pop the drum off and see if the shoes settle all the way back with the brakes released. Give 'em a jostle, they might pop back down. In that case, I would suggest lubricating the area where the equalizer bar rides, especially on the side where the parking brake lever actuates against it.
