Vacuum sources

On a 1986.5 928s which vacuum port is used for the vacuum modulator?
I see that there are four ports two in front and two in back on the intake block.

Walt

The only important thing is that the vacuum must be "manifold vacuum", not ported vacuum or other modified vacuum. That is, the vacuum must come off the manifold, not from the throttle body, nor thru any vacuum boosters, check valves, etc.