[RichardJ]

Another image showing ONE Combination Valve used to replace three independent units. Three independent units that operate, totally independent of each other, doing three separate jobs.

Second image shows an aftermarket Metering Valve. It has a dust cover and a pin that needs to be pulled OUT while bleeding. It doesn't push the piston or the shuttle valve in a Pressure Differential Switch Block.
The metering Valve has it's own job to do.

If there is air in the lines between the Metering Valve and the calipers, that air will compress and the Metering Valve will not open properly to allow bleeding all the air out of the lines. That is why the valve must be held open while bleeding.

The original style GM metering Valve is the large round item used on late 60's cars.

The Metering Valve opens with 50 to 75 psi, depending on application.