[RichardJ]

Still?

A Combination Valve is a COMBINATION of THREE separate valves. The three separate valves are COMBINED into a single unit to save all the plumbing necessary to get them connected and operational.

These three valves operate TOTALLY INDEPENDANT of each other.

This isn't rocket science, it's economics.

The first image shows the later style Combination Valve.
The second image shows a Metering valve, a Pressure Differential Switch, and a Proportion Valve.

The only difference between the first image and the second is the extra flared lines needed to connect them all together.

The pin, under the dust cover must be held OUT when manually bleeding this type of Combination Valve or the individual Metering Valve,

If the front lines are bled properly, this pin will push OUT, anytime you apply the brakes.

This style Combination Valve can be held out with a tool while bleeding.

The older style Combination Valve has a Metering Valve pin that must be held IN, while bleeding.