This sounds like a thermo expansion problem of the valves female NPT thread vs the male MPT thread nipple, not made tight enough? Teflon tape will always allow for a loose feeling even after a part has been torqued then can be retorqued again as it feels loose again. Overtightening is easy with teflon allowing over torquing and possibily splitting a fitting. Locktite is another solution that comes in different types but this I question against food grade items, I say don't use it. It tastes sweet like anti-freeze. The best sealer I would use is a sealer made by Permatex called The Right Stuff which is a elastomertic rubber compound. It comes out under pressure like a cheeze wizz dispenser a soft black rubber compound until it sets up. I bet when cured it will not produce any harmful toxic chemicals or compounds, just wipe off the excess after the part has been assembled before it cures.Give it a good wash then go mash with it.
It's for automotive use good against oil, coolant and ATF meaning it can stand high temperatures. When cured it's about as hard as a pencils rubber eraser. I bet this would lock the valve assembly in place tightly and leak proof for your MLT valve problem. I would clean the MLT sealing area with alcohol or lacquer thinner to remove the oils the cooler material is made of. Napa auto parts has it, not cheap like $11 even for a 7 oz container. I have used this with a 1/4" wide and tall bead on a flanged nylon tank 18" x 24" open top with a piece of glass lightly coated with Pam nonstick spray then placed lightly down against the bead lightly compressing it until cured. Removed the glass and cleaned it then lightly spring clamped it all around for a fish pond bio filter tank lid without leaks going on 5 years of outdoors use in the sunlight. I'm hard to impress and this product is one that works and impressed me. Not for head gaskets or in contact with gasoline, I think we're safe using it for homebrewing.
Bier Jagdwaffe... Bier 30 zeit.....~~=o&o>..........