Thanks Nate and Tom.
In the master rep, Select the components, right-click, set rep, Master.
That is exactly what I was doing BUT I was already in a user defined simplified rep, so it was setting the components in that rep to Master, not isolating as I was expecting. You have to be in the master rep for this to work.
I don't like working in the master rep on these models simply because they are large and unwieldy.