Metering valves are used on __________

Metering valves are specifically used in braking systems to control the brake fluid pressure to different axle sets of the vehicle, optimizing braking performance. In the context of the options provided, they are commonly found in front disc/rear drum brake configurations.

This is because front disc brakes typically engage before rear drum brakes to prevent rear wheel lock-up during hard braking situations. The metering valve allows the front brakes to receive fluid pressure first, ensuring that the braking action is balanced and providing better control and stability. This valve effectively helps in maintaining traction and avoiding potential accidents caused by premature rear wheel lock-up.

In contrast, metering valves are not necessary in systems where all four wheels are equipped with disc brakes or configurations that are designed differently, such as diagonally-split systems. In four wheel disc systems, brake performance is evenly distributed across all wheels, removing the need for a metering valve. Diagonally-split systems also do not rely on a metering valve due to their design, which distributes brake pressure differently. Hence, while options A, B, and D may refer to braking systems that serve different purposes, they do not accurately describe the specific application of metering valves as they relate to the front disc/rear drum design.