1. Metering Valve
2. Proportionating Valve
3. Combination Valve 1. Metering Valves A metering valve is designed to equalize braking action at each wheel during light brake application. A metering valve is used on cars with front wheel disc brakes and rear wheel drum brakes. The metering valve is in the line to the disc brake.
The metering valve functions by preventing the front disc brakes from applying, until approximately 75 to 135 psi (517 to 930 kPa has built up in the system. This over conies the rear drum brake return springs. 2. Proportionating Valve It is also used to equalize braking action with front disc and rear drum brakes. It is commonly located in the brake line to the rear drum brake.
The function of proportionating valve is to limit pressure at the rear drum brake when high pressure is needed to apply the front disc brakes. Thus the proportionating valve prevents rear wheel lock up and skid during heavy brake application. 3. Combination Valve A combination valve serves as two or three valves in one. It can function as a:
1. Metering valve
2. Proportionating valve
3. Brake warning light switch
Depending on design two or all of these valves are combined in to one housing. Many late model cars use a combination valve.
With some master cylinders, the proportionating and warning lamp valves are mounted inside the master cylinder housing. This design uses the same operating principles.