The proportional valve has two types: adjustable and non-adjustable. All OEM vehicles use a non-adjustable proportional valve designed to work properly with the specific front and rear brakes on the vehicle and the vehicle weight and tire size used. We generally recommend using non-adjustable valves only when the brake system is very similar to the factory system that the proportional valve is designed for, such as the disc/drum valves of many common GM cars shown here. For high-performance braking systems, such as those equipped with 4-piston or 6-piston calipers, we always recommend the use of adjustable proportional valves. The adjustable valve will allow the driver to increase or decrease the amount of rear braking force so that it can match the front. Once properly adjusted, the vehicle will stop safely without losing control or protruding the rear end from the side.