Rear brakes lock up, front factory disc 67 Barracuda

I added a proportioning valve that looks like the one Dano shows, other than the Wilwood name and yellow seal, plus mine is chrome. I recall $22 on ebay. Either from the same factory or those Chinese copy great. Usually the former and they run a night shift to make off-label parts.

SlantSixDan gives excellent info, as always, however I would add that for people who are in-tune with their car, an adjustable valve might make sense. There are many variables that can change the braking from the factory setup - different size or make of tires front & rear, different weight distribution from factory, different friction materials (very touchy on drum brakes as Dan relates). The factory proportioning valve was just to get it close enough, though looks like Chrysler messed up.

You don't want to adjust the proportioning valve for every trip (dead body in the trunk?), though braking conditions do change, but you may want to adjust after major design changes (bigger wheels in rear). If a circle-track racer, you probably would fine tune for every race. The downside to an adjustable valve is a potential to leak brake fluid, having something that a clueless mechanic could bugger with, and the temptation to keep tweaking it (some love that).