1998 379 air not building up

what valve is leaking (location)

 

Something isn't adding up. You write like you're doing this work yourself and you also list 17 years as a trucker.
I've never known any 17 year driver to describe things like "the the red arrow on the gauge drops to about 90 psi", rather than correctly saying "my secondary tank pressure drops to about 90 psi".

As you should know, once pressure is built up and you release your parking brakes (pushing red/yellow valves in), it will always "hiss" for about 5-8 seconds and drop in pressure a certain amount, to fill the trailer's air system. A 20~25 lb drop would not be unreasonable. However, if pressure keeps dropping, or you hear a leak beyond 10 seconds after pushing valves in, then you do have a leak/problem.