You can run both as they provide different functions.
The residual valves you already know about,whereas the proportioning valve provides balance between the front and rear circuits.
Due to the lighter rear end on most pick-ups when empty it's much easier to lock up the brakes in an emergency stop.
The proportioning valve (usually) reduces the pressure to the rears to prevent this.
There are adjustable ones available to allow you to tune your system to your personal needs.
