You NEED...to use heat.
Mine was leaking. I just figured to unscrew it, clean the old sealer out, and use a fresh thread sealer. Even though it had a sealer in the threads, unscrewing was actually VERY easy. Read that cheap aluminum ! Anyway, yeah, removed ALL of the body's aluminum threads and left them in the fitting.
I went up to a 3/8" NPT and plugged it.
I also had to use a very high pressure sealer to get it to seal. But after many trials, it's now sealed and will hold the whole 300 bar.
And YES, you NEED to remove ALL of the air pressure before the removal process.
Mike