The switch turns on the brake lights when you put on the air brakes. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front wheel braking is good under all conditions. Tests have shown front wheel skids from braking are not likely even on ice.
Many vehicles have automatic front wheel limiting valves.
These valves cannot be controlled by the driver. All trucks, truck tractors, and buses must be equipped with emergency brakes and parking brakes. Spring brakes are usually used to meet these needs. When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes.