well? I know it isn't WD-40. Torch didn't work either.

Kroil, the oil that creeps.

GIBBS spray...it's ridiculous stuff!

I've had good luck with PB Blaster, but when it comes to old, rusted brake fittings, they often weld themselves together so that nothing can get them unthreaded.

I took a "toothbrush type" wire brush and knocked loose any external rust so that the penetrant could soak in. I then used a mix of 1/2 Acetone and 1/2 trans fluid and let it soak. I was able to break loose 13 45 year old brake lines and reuse the original lines.

Let it sit a day or two and keep squirting it in.

propane or acetylene? You might put the acet (sparingly) on it then shoot it while hot with some WD40 or whatever you have & see if it works

heat the fitting and/or around the casting. then apply a birthday candle, allowing the wax to generously melt against the hot surface[s]. the wax will wick into the joint[s] like solder does when soldering copper piping. you may be pleasantly surprised at the results. you may have to do this a couple of times though. i remove all of the 5-40 slot head screws on the cowl vent screen on my 33 dodge humpback panel truck using this method, even though many of them had badly messed up slots in the screw heads. all but one turned out as if they were installed yesterday and were new.

Mix ATF with acetone 50/50. Way better than kroil.

I'll give it a try since that stuff is nowhere to be found (except by mail) plus it is EXPENSIVE.

Home Depot has acetone, plus I have a can of it at my house, right around the corner from you. Come and get it.

I often get the threads freed up but the line is fused to the nut and spins.

If they don't come loose its not worth fighting too much IMO. Nip the lines off with a bolt cutter and bend up new when it is time to put it back together. Easier to get the nut off on the bench in case you are saving the wheel cylinders or calipers. Most of the time they need rebuilt or replaced anyhow.