[Gav]

Spooky,
I use what is called a monometer for measuring very low levels of pressure or vacuum with work.
You can buy a digital one that will be accurate to within .01 of a pascal for about $300 from System Control in Brisbane or Farnell.
However if you don't need to know the measurement but are just interested in ensuring that the vacuum is equal across all 8 trumpets you can make a slack tube monometer from a peice of wood and clear plastic hose.
You will need to fix the hose in a U shape about a meter down then up and fill the U shape portion about half full with water.
By waving the open end of the hose in the mouth of each trumpet, the vacuum will raise or lower the water level in each side if the slack tube. You could note where each is with a texta mark or set them all to one mark.
3/8 tube would work fine.
Give me a ring if need be, 0407 110641

Gav