6,000 P.S.I. air thread seal

I have a metal High Pressure air cylinder which developed a small leak at the threaded connection to the brass valve. I've seen Loctite 545 and RectorSeal #5 thread sealants, but their rating is 3,000 and 2,600 PSI. Any recommendations on sealants to fill to 4,500 PSI? Thanks Lupe G.
Several years ago a Hydraulic Jack repair shop told me they used LocTite 518. I have been using it for a while now. I did try several other Loctite products before ..242, 539, 545, not much luck. The 518 is easy to find and reasonably priced..it's meant to be a Anorobic gasket maker. Just be sure to get everything real clean, apply to the male threads, assemble, and let cure for 24hr's.