Nick,Dead giveaway is having a vacuum line to the pressure regulator that eased pressure at idle. Opening the TB would decrease vacuum so the pressure would go up.
I just checked the MAP sensor using the procedure listed in the link below - I know it doesn't list a 7.5L motor, but I think the test is the same:Vacuum at idle in Park = 19"
Vacuum at idle in Gear = 17.5"
Vacuum at 1500-2000 rpms (no tach so just guessing here) = immediately drops to 13" when I first touch gas pedal then bounces to 20-21"