I think it may be the grommet where the one way valve plugs into the vac booster. When I tried pumping it down when the hose was connected to it I couldn't get it to hold anything. I popped the one way valve out along with the hose and pumped it down to 25" no problem. I plugged it back in and with a crap load of pumping with the MightVac it pumped down to 20" and seems to be holding. I'm going to let it sit there for a while and see if it held.
'96 328iC "Marie"
R.I.P 14 November 2013