BRASS COMPRESSION SLEEVE 3/16 INCH

Per the supplier: First, you should be using the 'TH' series sleeve that is made for nylon air brake tube. In your example, TH260B. The nut slides on the tube, followed by the sleeve. Next, slide the fitting over the tube and over the sleeve. Move the nut over the fitting and tighten.

Per the vendor, this will not work with 1/8 line. You would want to use a 261UB-02 fitting for 1/8th.