Anything below 1bar (or 14.7lb/sq.in) is vacuum or negative pressure as opposed to atmospheric pressure.
Your handpump is the tool you need.
IF you ever find the solution to your problem, pls. post it here.
I have been chasing the very same problem on my Benz for months and have spent so far over $1000. in diagnostic to find the cause.
My dealer and independant mechanic both agree there's a problem but neither can find the cause.
I've posted this problem twice on this tread but did not get any result so far.