In vehicles equipped with a disc-drum brake setup, a key component that controls the pressure specifically to the front brakes is the metering valve.
Understanding the Metering Valve
A metering valve is a specialized hydraulic valve used in braking systems where the front wheels have disc brakes and the rear wheels have drum brakes. As stated in the reference, the metering valve is installed on the front brakes.
How the Metering Valve Works
The primary function of the metering valve, according to the reference, is to regulate the hydraulic pressure reaching the front disc brakes. It achieves this by holding off pressure to the front disc brakes for a brief period. This delay is crucial because it allows the slower-acting rear drum brakes time to activate and catch up with the quicker-engaging front disc brakes.
Why This Control is Important
The difference in activation time between disc and drum brakes necessitates this control mechanism in mixed systems. Disc brakes respond almost instantly to hydraulic pressure, while drum brakes require pressure to overcome the return springs and force the shoes outwards to contact the drum. Without a metering valve in a disc-drum system, the front brakes would engage much faster and harder than the rear brakes during light to moderate braking.
This imbalance can lead to:
- Excessive front brake wear
- Instability during braking
- A tendency for the front end of the vehicle to dip sharply
By delaying the front pressure slightly, the metering valve helps the front and rear brakes apply braking force more proportionally, especially at lower pressures. This coordinated action helps to improve braking stability and, as mentioned in the reference, assists in preventing nosedives—the undesirable forward pitching motion during braking.
In essence, while the master cylinder generates the initial hydraulic pressure for the entire system, the metering valve acts as a pressure regulator specifically for the front brakes in disc-drum configurations, ensuring balanced brake application between the front and rear axles.
