Metering valves are installed on vehicles that have front disc rear drum systems. This is because front disc brakes apply almost instantly while the rear brakes must overcome spring pressure. Without this valve the vehicle would nosedive. In fact nosediving is a symptom of a faulty metering valve. Applying only the front brakes may cause vehicle instability and skid.
To achieve balanced braking, the metering valve is held closed by spring pressure to delay application of the front brakes, allowing the rear drum brakes to overcome retracted spring pressure and engage just before the fronts. When the brake pedal is released, fluid is allowed to bypass the valve and return to the master cylinder.
Open the bleeder screw to a front brake caliper and have a helper slowly apply the brake pedal. There should be a delay in flow from the caliper bleeder. This is because the metering valve is holding off pressure until the rear drum brakes have applied. It’s best to use a pressure kit to test this delay and measure actual pressures. Connect a pressure gauge to the inlet and another to one of the outlet ports.
If air gets trapped in the brake line between the master and the valve, it can be difficult to bleed the brakes. This is because air is compressible and pressure may not build high enough open the valve. Metering valves are typically located in the combination valve. The stem at the end of the valve is for bleeding any air trapped by the valve. A small clip is used to open the valve and allow aerated fluid to bypass the valve and exit the system.