The cap is to block off a line, so will probably not normally be needed, and the insert is to support the tube from collapsing when the nut is tightened. Dwg 36 for RV-7 has a diagram for how to assemble. If using high pressure Parker tube there is no need for the insert. The various brands of tube have their own inserts, the diameters may vary slighty.
Paul vS (yes I'm also a Van)
Building RV-6A #22320 O-320 FP. Wings and tail complete, fuselage almost done, working on canopy.
Flying my Aeroprakt A-22 STOL and the aero club's RV-9A while I build