2500 rear brake question

left rear, frame mounted, lever operated brake controller, connected to axle with a rod.
Looks like it is activated when weight is in the box. The brake lines on it are rusted and one is leaking. What is the term to describe this thing?
Do 1500's also have this or is it for heavier models?
Thanks any info.

It's a proportioning valve, "hight sensing valve controlls brake pressure to rear wheels based on weight of load. TSB, http://dodgeram.info/tsb/1998/05-04-98.htm