I use Loctite 567 thread sealant. Just fill the threads on the male fitting making sure none gets on the end or in the hole, thread it in and tighten it. Use it on the male pipe thread manifold end of the fitting and sensor.
The Loctite 565 or 567 goes on the end of the elbow that threads into the manifold only and not the other end that connects to the hose. A small quantity of sealant should be used to fill the male threads all the way around the fitting, but not on the first two threads, leave these dry. Tighten to finger tight and then 1.5 - 2 turns more to get it clocked correctly to meet the hose.
Sealant on the flare side doesnt allow the 2 opposing flares to merge. Sealant could eventually ooze out of the connection, leaving a small gap, creating a leak.
Flare side? My comments were only intended for the the pipe threads. If your flare connection is leaking, adding Loctite is not a diligent remedy approach. Never heard of nor would accept such application on the flare. and I doubt the vendor would approve their product for such an application.
In a flare fitting (37 degree AN type), the seal is achieved by direct, precision contact on that 37 degree face. The threads of the B nut or nipple are not designed to be in the leak path of the system - they merely provide the mechanical force to clamp the sealing surfaces together. There is no sealant required, and even in cases of scratching on one of the faces, that can often be saved with “seal savers”. These are a sacrificial metallic “gasket” installed dry.
The reason that loctite doesn't go on the end that connects to the hose is as others have stated above. Re. tightening the coupling "B-nut" (AN818) on the hose/tube, it is torqued to 50-65 inch pounds for a -4 aluminum flare and 135-150 inch pounds if the -4 flare is steel. This reference info is in AC43.13-1B Chapter 9 Section 2.
