[RichardJ]

I don't understand why the English language is such a challenge for some on this board. Actually, not just this board, but throughout the automotive world.

com·bi·na·tion
/ˌkämbəˈnāSH(ə)n/

noun: combination; plural noun: combinations
1. a joining or merging of different parts or qualities in which the component elements are individually distinct.
"a combination of blackberries, raspberries, and rhubarb"

An automotive COMBINATION VALVE is THREE valves COMBINED into ONE unit. It is comprised of a Metering Valve, a Pressure Differential Switch and a Proportion Valve. Each of those three items serve different functions.

A Pressure Differential Switch that Andy showed you is a stand alone unit, that turns on the dash warning light and nothing else.
A Pressure Differential Switch in some form, is used on dual brake systems with drum/drum and disc/drum and disc/disc. It is used with power and non-power systems.

The first picture is a Combination Valve.
The second picture is the exact same thing, but as three individual units. This requires a lot of plumbing.