Glen, Your description of the problem is correct. I did try doubleclick with the Break Line tool and it added a node instead. Tried converting the Molding to Plain Polyline and doubleclicking with Break Line and had same result.
Alaskan_Son, it's a really big/complicated plan so probably easier to describe. I created a swimming pool from a closed polyline using a series of terrain features, etc- including specifically a collection of moldings extruded on the inside of the polyline. Since several objects now have the same boundary defined by that polyline, I put the moldings, and lines intersecting it where I want to cut, and a separate temp layer to facilitate access in planview editor. I've tried Trim tool (using the intersecting lines) as well as the Break Line with double click. Trim tool told me I couldn't trim the object- said the same after I converted it back to a plain polyline.