i do believe that you are talking about the valve near the rear end in the brake system. it is there to increase the brake pressure in the rear brakes based on load in that end of the vehicle, there fore decreasing the load on the front brakes.
This is called a load sensing proportioning valve It is designed to reduce the rear brake pressure depending on the amount the rear suspension is unloaded (this prevents rear wheel lock-up that could result in loss of vehicle control and a crash!)
