Duster proportioning valve help!

Mother Mopar had a one fits all attitude with proportioning valves and as a result most all A bodies have suffered from less than perfect front to rear proportioning, even when new.
This usually results in the rear locking up and causing the vehicle to want to spin like a top. Use of an adjustable proportioning valve inserted in the rear brake line allows you to fine tune the rear brake lockup threshold, making the car far more controllable under hard braking. Depending on what you start with you may, or not, need to "gut" the proportioning valve section of the distribution block to make it work properly.

Summit's house branded one was the least expensive one I found the last time I bought one, but there's been lots of new products put on the market since then.