I see.

The Schraeder valve is normally self sealing by the rubber ring below its threads and the spring that keeps the valve bottom shut (another rubber seal there). When I installed mine I put Nylog blue on the rubber ring (the one below the threads), the threads itself (yes) and another dab at the valve bottom (have to press the valve tip a bit in order to expose the bottom rubber seal to apply). Nylog blue is available thru ebay and maybe local your AC shop uses it.

Other than using Nylog I think you could try to retighten the Schraeder valve using a valve core tool. Firstly loosen it a bit (not too much as refrigerant could escape - this just to know how tight it was installed) and retighten it tighter than that. If you retighten it during AC is running (engine on, etc) then the low side pressure should be around 35 to 45psi (compared to AC off at rest around 80 to 100psi) so that not much refrigerant could escape if it does.

You could apply the soap bubble test (dishwashing soap + water, worked up with sponge until lots of bubbles) on top of the valve body opening after retightening. Tighten a bit more (using valve tool) until no bubble pops out.

Last time I did also install that second O ring within the valve cap (one suitably sized, located around the inside threads) but I find this is not needed with the outside O ring on valve body renewed.

The Nylog is worth trying if retightening wouldn't work (but this would require you to evacuate the system and recharge).

I think thats all to it.

Good luck!
Amarin.