We broke two 1/4" drive ratchets off in one before we discovered the technique below.
The way to get that baby off is with a hammer-driven impact driver and the right combo of adapters to get to 1/4" (mine has a 1/2" drive, so I use a 1/2" -3/8" and then a 3/8" to 1/4" adapter). I've seen those drivers in 3/8" drive at the autoparts place, which would eliminate one adapter.
Squirt on some penetrating oil at the seam around the cap, let that soak for a while. Then heat the fitting below the cap with a mapp torch or whatever torch you have. I used a little butane one successfully. Be careful to not heat so much you blister the paint off. Also try to keep from heating the cap.
Then a few pops with the impact driver and she should loosen right up.