I have created an assembly in which two bulkhead compression fittings have been positioned/mated in appropriate clearance holes through plates separate plates. I have successfully create a simple tubing part (extrude of an ID and OD) which runs between the two fittings and is dynamically linked such that when I move the fittings, the hose part changes.
In order to achieve this my workflow was: I created a part-in-place, within the assembly. I then created a 3D sketch that would be the axis of the tube. This line started and ended on surfaces found on the fitting corresponding to the maximum insertion depth of the hose or hose end fitting. Finally, a 2D sketch, again placed on a plane corresponding to the maximum hose insertion depth, allowed me to do a swept extrude. I stored the part file externally.
I would like to do the same, but create an assembly. This would allow me to include hose end fittings and build the complete hose assembly model. The crux of the problem I would describe as follows: Within my main assembly (has the fittings I'd like to reference) I do Insert -> Component -> New Assembly. I then create a new part corresponding to the hose body within the new assembly. While editing this part, it seems I cannot reference surfaces/features in the main, highest-level assembly.