I believe what you are referring to is the rear brake proportioning valve. There is a thin metal rod mounted to the rear axel which extends upwards to the proportioning valve which is located along the frame rail. As the weight in the back of the bed increases/decreases, the rod moves up/down, which adjusts the amount of rear braking... Is this what you are referring to???

1984 Toyota 4x4 Truck, 4.88's, 32x11.50's, weber 32/36, aux gas tank, Warn 9500i, SR5 guages, tilt/altimeter, Alpine stereo with dual 12's behind the seats.