The "hose inside of a hose" (HIH) carries the fuel line inside of another hose filled with hot engine coolant. It is terminated useing compression fitting on a copper T. The fitting
tightens down on an O-ring which seals around the fuel line.
Here's another view of the HIH
This HIH was made by adding a coupling to the end of the copper T and threading it. It's probably better to use a compression fittings that's made to solder onto 1/2" copper.
The solder-on 3/8" compression fittings, sitting next to the HIH, work well when useing 3/8" O.D. for copper fuel line.
Sometimes the O-ring doesn't seal well against the rubber, which can shrink away from it. This can also cause the rubber fuel line to have a smaller I.D.
This hose was removed when I switched this section to 3/8" copper fuel line. The copper allows the O-ring a better seal.
Most of my system is still 1/4" rubber. It doesn't leak. This section was a little stressed on the end causeing the fuel line to pull away from the O-ring.