After a suitable amount of time with no pressure, & a good cleaning with alcohol or acetone, you could epoxy the cap on with JB weld.
I know it sounds a little "bandaidish" but I fixed a leaky banjo fitting (fuel) with the stuff two years ago in Moab, & it still no leaky to this day...
I've never looked closely at the schrader valve thingy on the fuel rail, I always assumed (apparently incorrectly) that the innards came out with a valve core removal tool...![]()