Brake Combination Valves
A three function combination valve contains the proportioning valve, metering valve, and the pressure differential switch in one convenient location, typically the firewall. A two function combination valve omits either the metering valve or the proportioning valve. Let's have a look at the three functions of this three in one 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 is used 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 temporarily reducing fluid flow and pressure. When the pressure builds to a predetermined amount (75-100psi) flow and pressure to the front calipers are returned to normal.
Proportioning Valve: The proportioning valve is used to prevent rear wheel lock-up. These valves achieve brake balance by reducing fluid flow and pressure to the rear brakes during hard and sudden stops. This valve helps achieve better braking only during hard and panicked stops. Most of the time, during light and moderate braking the valve lies dormant. Without the valve the rear brakes would tend to lock first because the vehicles inertia transfers the weight from the rear to the front while braking.
Pressure Differential Switch: The pressure differential switch is used to inform the driver if there is a leak in the system resulting in a loss of pressure. It is used in dual master cylinders to detect a difference in pressure between the two sides. If there is a difference in pressure it will complete a circuit and illuminate the red warning indicator lamp.
