Proportional valves exist in vehicles that have disc brakes in the front and drum brakes in the rear. Without the proportional valve, the disc brakes would be applied before the drum brakes because the pads in the discs are in contact with the discs, but the shoes in the drum brakes don't.
In the presence of proportional valves, drum brakes are applied before disc brakes. This helps control braking and stability.
Proportional valves do not allow pressure to reach the disc brakes until a predetermined pressure is reached, which is below the maximum brake pressure, thus allowing the drum brakes to be applied first.
Since it requires less pressure than the front brake, it allows less pressure to reach the rear drum brake.