Brake Combination Valve
Metering Valve: The metering valve is in charge of delaying the application of the front brakes just long enough to overcome the rear brake springs. It's installed on vehicles with front disc rear drum braking systems. When the brakes are first applied, pressure quickly overcomes a spring in the valve and closes a port to the front brakes. This temporarily reduces fluid pressure. When the pressure builds to a predetermined amount (75-100 psi.). After this brief delay, pressure to the front calipers is returned to normal. When a metering valve fails, the vehicle's front brakes apply early and the tends to nose dive.
Proportioning Valve: The proportioning valve is used to prevent rear wheel lock-up. These valves achieve balanced braking by reducing fluid flow and pressure to the rear brakes during hard and sudden stops. It activates only during hard and panicked stops. Most of the time during light and moderate braking the valve lies dormant.
Pressure Differential Switch: The pressure differential switch is used to inform the driver if there is a leak in the system. It is used in dual master cylinders to detect a difference in pressure between the two sides. If there's a difference in pressure, the switch will complete the circuit and illuminate the red brake warning indicator lamp.