Hi dibdub.
The valve is a load compensating valve for the rear brakes, the more that you load up the car, the lower it will sit and there is a spring on the rear axle that is connected to the arm on the valve. The spring pulls the arm alowing more brake pressure to the rear brakes and more braking effort. Located on the n/s/r of the car, 4 pipes on it. Probly siezed, drown it in wd40 or similar, and work the arm back and forward. Check opperation by getting someone to press the brakes and the arm should move.
Steve.