Is this gas cap vented
.......... weld a nut to the side of the filler neck over the hole you drilled. Take the flared end of the brake line with the flared fitting and thread it into the nut. ..................
That will not seal..................
But it's headed in the direction of one "right path."
Options:
Weld a tube direct to the filler as I mentioned earlier
Go ahead and use a nut but do use a 3/8 X SAE fine thread. Buy a 1/8 pipe tape and chase the nut with the pipe tap when you are done.
Then buy a "half union" with 1/8" male pipe on one end, and the correct inverted flare end to fit your tube you are using, such as 1/4" brake tube
The brake tubing fitting MUST have the proper inverted flare seat to seal.
This is what you want below: 1/8 pipe on the male end, whatever angle you desire for the fitting, and the proper size female inverted flare to fit the tube
If you have the knowledge and equipment to silver braze you can also either directly braze the tube into the filler, or you can braze the adapter fitting to the filler.