I'm not particularly familiar with the setup used in the 760, but there's
two things that should prevent this, one is a checkvalve, though I don't
know where it's located for sure but on a 240 it's right off the intake
manifold and the other is the electric vacuum pump which is under the hood
on the inner fender. If this pump is running then you may have a vacuum leak
somewhere preventing it from pulling a vacuum when there's no engine vacuum.
