Chris, lemmie share a fire riser story with you.
many many years ago. late 70's/early 80's. Ronnie [my training plumber] and i
fixed a leak on a 3" fire riser main. it was all screw pipe. no vic pipe in them days.
when we got finished, and turned the water on. no leaks. GOOD!
but, when we turned on the jocky pump. we would get a very small seepage where new tied into old.
so, we tore it apart, redoped/ added teflon tape. put it back.
damn thing leaked again, again only when pump was turned on.
this time, we tore it down, wire brushed the female end, chased the threads male end, teflon tape and dope
SOB still leaked.
the boss came by, to see how we were doing, and my plumber explained, it would barely seep only when pressuer up
above 70 psi
he handed us some lead wool, and a small blunt screwdriver. we packed that joint. problem solved