My 88 760 T-wagon has 2 vacumn lines coming off the plenum, one white, one black.

My a/c vents close under turbo boost. I have checked with a Mighty Vac both check valves, replacing one (black line) which had with a very slow leak. The vac shows a hold of vacumn on the white line after much pumping...must be vacumning the resovoir. It holds good at 25 lbs. The black line will not hold a vacumn. Is it supposed to???

FAQ's said somehting about the black line going to the dome lite to measure something. I checked the dome lite, I don't see any vacumn line up there only
electrical. Does anyone know where the black line goes??