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You should have just one model for the valve assembly. This valve model should allow you to position the sub-components to achieve the open / closed state. These states can be captured using the drag bodies / snapshot tool. The saved snapshots can be re-used in snapshots of higher level assemblies, and for visualization in the valve's drawing (if made accessible as exploded states)
When you use the word "instance", everyone is going to assume you are already using a family table.
You have options:
1. Use a family table "instance" and add the dimension that controls the valve open/closed.
2. Use flexible component to control the dimension in the assembly.
3. Or as pausob described using states.