On my '83, I can hook up a hand vaccum pump to the mechanical vaccum on the distributor, pump it and the vaccum will move slightly and go back to the normal position. Is this normal, or is it suppose to maintain it's position until the vaccum is released?

 

it has a spring I beleav so the advance plates return under no vacum.

 

As long as you have a reading on the gauge, the plate should hold position. If it leaks down your not getting full advance. But make sure your pump doesn't leak.