"Mike Noel" wrote in message
. ..
OK, we now know the problem was a pinched hose, but there is some
speculation that a failed vacuum pump can actually blow dust backwards
into the vacuum instruments. The idea is that residual vacuum at the
instruments allows the higher pressure at the failed pump to push the dust
into the vacuum instruments. There is an STCd filter that can be placed
between the instruments and the pump designed to prevent dust backflow if
the pump fails.

I thought vacuum systems had a check valve that prevented backwards moving
air. No?

If not, it sounds like they should.
--
Jim in NC